Sometimes Nikki Nachampassak looks out the window and watches as potential customers pass by her Portsmouth restaurant and mouth the words “banh mi.”

“They have this look on their face like, ‘What is a banh mi?'” she said.

Nachampassak owns Nikki’s Banh Mi, where she sells several varieties of the Vietnamese sandwiches, as well as banh mi tacos and a banh mi pizza. Classic banh mi are made on an adapted French baguette, spread with pâté and mayonnaise and filled with Vietnamese cold cuts, herbs, pickled vegetables, and peppers. Once Nachampassak’s customers taste the saltiness and sweetness of the meats, the crunch and color of the fresh herbs, and the tanginess of the pickled vegetables, they’re hooked, she said.

“It’s definitely a younger crowd that I see come into the shop,” she said. “But I also see an older crowd that are a little adventurous, in their 50s and 60s, and are like, ‘My wife told me to come in and get this.'”

In Portland, it used to be just the foodie crowd who knew what a banh mi was, and that you could get one at Kim’s Sandwich on St. John Street, before it closed in 2016. Today these Vietnamese sandwiches seem to be everywhere, showing up in both traditional and experimental forms. In Portland, the Mellen Street Market sells the original Kim’s sandwiches now, but in the form of a panini. Banh mi are on menus at Asian shops and restaurants, such as Veranda Asian Market, Saigon Restaurant, Sun Bakery and Cafe, and Huong’s Vietnamese Restaurant, but they’re also showing up in less expected places, such as the Thirsty Pig, Eaux, Otherside Deli, the Honey Paw, and the Blue Rooster. Several variety shops in the Portland area offer the sandwiches as well, and many food trucks, such as the Biddeford-based Farm To Coast Mobile Kitchen, sell them.

What’s behind this local sandwich explosion? In part, the price. Banh mi often sell for $5 or less, while some other sandwiches in the Portland area cost twice that amount. Also, the banh mi is one of the fastest-growing items on restaurant menus in America, according to Datassential, a Chicago-based market research firm that tracks sandwich trends. The banh mi is found on just 2 percent of all restaurant menus in the United States, but its presence has grown a whopping 70 percent over the past four years. The sandwich is expected to be on 2.6 percent of U.S. restaurant menus by 2022, outperforming 95 percent of all other food and beverages, according to Datassential.

Kathryn Shear, a senior analyst with Datassential, says her generation of millennials and members of Generation Z – those born from the mid-1990s to the mid-2000s – are fueling the rapid expansion of the banh mi.

“We’re more adventurous in trying these new flavors,” she said. “Sandwiches are a very approachable format for new, adventurous eaters. (The banh mi) is something that’s familiar, but it has all these new flavors. It has cilantro and Thai basil.”

But local restaurateurs say the interest in this region at least partially crosses generational lines. Tuyet Thi Le, who plans to open a banh mi shop on Cumberland Avenue in June called Banh Appetit, says age was not a factor at her mother’s restaurant, Huong’s, on St. John Street. “We would get all ages coming in and trying it, even little kids,” she said.

Caleb Mason, a 63-year-old writer from Portland who publishes under the pen name Don Trowdon, experienced his first banh mi in New York City and now seeks them out in Portland, most often at Sun Bakery and Cafe on Forest Avenue. Mason says that, for him, ordinary sandwiches have “become kind of boring,” and the texture and well-balanced flavors of a banh mi makes for “a nice change.”

“It is a definite craving,” Mason said. “It’s a food that, when you want it, you really want it.”

Banh mi are a fusion of ingredients from two cultures, a result of the colonization of Vietnam by the French in the late 1800s, when Vietnam was part of French Indochina. The French brought baguettes, lots of meats, and pâtés to the table. By the 1950s, the Vietnamese were fiddling with the sandwich, making their own fluffier version of a baguette and replacing some of the meat with crunchy fresh and pickled vegetables and pungent herbs.

A classic banh mi, Le explains, begins with Vietnamese French bread. (No hoagies or hard, crusty French baguettes, please.) Le plans to buy the Vietnamese bread for her new shop in Boston so it will be as authentic as possible. Some swear that it’s a touch of rice flour that makes the Vietnamese loaves so light and fluffy; others say no, it’s steam in the oven. Whatever the trick, it makes a difference.

“It’s a lot different from the regular French baguette that you get from the local bakeries around here,” Le said. “It’s more light and airy, and it has a nice crisp when you bite into it.”

Inside a traditional banh mi, Le says, you’ll find Vietnamese sausage made with fish sauce and pork, Vietnamese ham, also known as head cheese, mayonnaise, pork pâté, pickled carrots and daikon, cucumber, cilantro and red chili peppers.

“Nowadays we just put jalapeno peppers in there, and that’s just as good,” Le said. The sandwich then gets a light drizzle of soy sauce, she added.

Nachampassak says pork roll – steamed pork sliced like a cold cut – is also a must-have in a classic banh mi.

Le will be serving the traditional banh mi at her new shop, but, like a lot of other places, she’ll be modernizing the sandwich as well, making, for example, a beef-lemongrass banh mi, a version with Korean barbecue and kimchi, and a tofu banh mi with a vegetarian fish sauce.

Other local restaurants are also switching out ingredients in the sandwich, and some of the more interesting mashups fuse banh mi with food from other cultures. Bird & Co. in Portland sells banh mi tacos, for example, and Gather in Yarmouth has a banh mi pizza on the menu.

Otto featured a banh mi pizza as a month-long special in April. It was made with hoisin pork and pickled vegetables, and was drizzled with a cilantro-sriracha aioli. The company sold more then 5,000 slices and just under 1,000 whole pies in its Maine and Massachusetts locations, according to spokesman Eric Shepherd. For a monthly special, he said, the pie “fared quite well.”

Wills Dowd, co-owner of Bird & Co. in Portland, ate a banh mi about once a week when he lived in California. “Even in Portland, I’ve seen it’s becoming more and more popular every week,” he said. “Places that never used to have it now have it.”

The banh mi taco at Bird & Co. is one of many such mashups at local restaurants. Ben McCanna/Staff Photographer

The restaurant’s banh mi taco is one of its more popular fusion tacos, he said. It contains Vietnamese pork shoulder that’s been marinated in lemongrass, fish sauce, soy sauce and honey. The toppings are pickled radish, pickled carrots, cucumber, cilantro, sesame seeds and spicy mayonnaise.

Nachampassak also serves banh mi tacos, and her banh mi pizza is a kind of secret menu item – it’s not actually on the menu because it was an experiment, but she’ll make one if you ask for it. Instead of tomato sauce, she brushes the pizza dough with garlic butter, then adds pizza cheese and meats. She puts it in the oven to cook, then after it’s baked adds sriracha mayonnaise, cucumber, pickled carrots and cilantro on top.

The banh mi breakfast sandwich at Mason’s Brewing. Photo by Jeremy Smith

Mason’s Brewing Co. in Brewer features a banh mi breakfast sandwich as a rotating brunch special. Bacon, egg, pickled carrots, bacon jam, green tomato kimchi and mayonnaise nestle inside a ciabatta roll. Another of the brewery’s banh mi sandwiches is closer to traditional, featuring pulled pork, sliced pork roll, bacon pâté, red cabbage kimchi, whiskey-pickled jalapenos, cucumber and mayonnaise. Jake Bridges, Mason’s restaurant manager, likes to call dishes like these “remashed” mashups.

“Isn’t that cool how food changes, and different cultures come together and it creates this new thing?” Bridges said. “It’s like languages. Food is the same thing. It grows and changes.”

What happens when a classic Vietnamese sandwich meets a lobster? This. Boy oh boy! Photo courtesy of Maine Lobster Marketing Collaborative

Banh Mi–style Maine Lobster Roll 

We think this lobster roll-banh mi mashup is just the thing for summer. The pickled vegetables need to sit for an hour before they’re ready, so plan ahead. Recipe courtesy of the Maine Lobster Marketing Collaborative.

Serves 4

1 cup matchstick-cut carrots

1 cup matchstick-cut radishes

2 cups white vinegar

1 teaspoon salt, plus extra for salting the vegetables

1/4 cup sugar

1 cup coconut milk

1/2 cup mayonnaise

1/4 cup fish sauce

1/2 cup chopped fresh mint

4 cups cooked Maine lobster

4 Vietnamese-style baguettes, cut open

Butter, for toasting

Cucumber, sliced

Cilantro, chopped

Thai chili peppers, sliced

To make the quick-pickled carrots and radishes, toss the vegetables in a colander with a lot of salt. After 30 minutes, drain and rinse them. Mix the vinegar with the 1 teaspoon salt and sugar in a bowl or liquid measuring cup and microwave for about 2 minutes until the sugar and salt dissolve. Pour the hot vinegar mixture over the carrots and radishes and let sit for an hour or so.

To make the sauce, combine the coconut milk, mayonnaise, fish sauce and mint in a small bowl. Add the lobster to a large bowl and pour half of the sauce over it. Combine until the lobster is lightly dressed. Add more sauce as needed, but use a light hand.

Butter the rolls on the outsides, then put them briefly in a skillet on the stovetop, outside side down, until the first side is lightly toasted, flip and toast the second exterior side. Remove from skillet and fill with cucumber, cilantro, chili peppers, reserved pickled carrots and radishes, and lots of lobster.