Consider the grilled cheese.

As ubiquitous and American as tomato soup, it's pure comfort to the core. A dish as universally satisfying as it is basic, the grilled cheese's juxtaposition of texture — a lightly crisp, buttery exterior yielding to a molten, slightly salty interior — draws lifelong fans.

It's no wonder, then, that National Grilled Cheese Month (April) inspires such intense fervor. In New York City, the love for grilled cheese manifests itself in the form of more than a few restaurants devoted solely to the sandwich. There's the Melt Shop with sandwiches so popular that the restaurant had to open a Chelsea location. And there are grilled cheese trucks aplenty — The Morris Truck, Milk Truck and Food Freaks among them.

Fortunately, plenty of online resources are available to help suss out the best grilled cheese from coast to coast. Esquire's food blog hails the mighty grilled cheese from The Big Apple to Tokyo. Major food authority Zagat is also getting into the game, with its bloggers helping the faithful nail down the best grilled cheeses in big cities such as Los Angeles and San Francisco.

So, what's behind the power of grilled cheese? “It's really very simple,” says Connie Fisher, who owns The Pop Shop in Collingswood, N.J., with her husband, Stink. The restaurant serves more than 30 varieties of grilled cheese, from the basic to the bold.

“I think people usually start loving the simple things, and it gets more exciting as it gets more complex,” Fisher says. “But you always remember the simple part, like that your mom made you a grilled cheese when you were little … it brings back all those feelings, but it's good too.”

Throwdown at the Throwback

Opened in 2005, The Pop Shop has a retro-diner feel. An old-fashioned soda fountain serving Cokes and floats with extra fizz and flavored syrups gives the restaurant a throwback drugstore vibe. All that's missing is the soda jerk.

Home to an extensive menu boasting more than 200 diner standards, this mom-and-pop soda shop was once featured on the Food Network's Throwdown with Bobby Flay. The battle du jour? Grilled cheese.

“It was so flattering, really thrilling,” says Fisher.

The couple battled Flay's sandwich with The Calvert, which gets its name from a street in the borough. The Calvert has roast turkey, avocado, bacon, Monterey jack cheese, tomato and balsamic mayonnaise piled on focaccia.

Even though the Fishers lost the battle, they remain graceful in defeat. Flay's sandwich, the 5 Bs, features brie, goat cheese, bacon and tomato on country white, and remains on the permanent menu. But, as far as the customers are concerned, the winner seems to be The Calvert.

“Ours really outsells his at least two to one every year,” Fisher says with a laugh. “There's a little satisfaction, you know?”

‘We Take Grilled Cheese Very Seriously’

Every April, The Pop Shop celebrates Grilled Cheese Month with a daily grilled cheese special and other fanfare. “We also have people nominate a grilled cheese king and queen,” Fisher said. “That goes on all month, and then we crown them... and they win a year's worth of grilled cheese.”

Since April is National Poetry Month, too, there's also a cheesy poetry contest. “We take grilled cheese very seriously,” Fisher adds.

And so do some of The Pop Shop's clientele. The restaurant's menu, which includes everything from the very basic bread and cheese to the more adult Frazer with its jalapenos and spicy chipotle mayo, brings pilgrims from all over in search of the holy grail of grilled cheese. “They put us in their vacation plans because of the show,” Fisher says. “It's really kind of crazy.”

Sixteen men and women traveled on a bus to New Jersey from Indiana just to try the grilled cheese. One couple flew from Boston and back in one day. “They had lunch and dinner with us, and then they flew home,” Fisher says. “But if you're going to do something, it should be all the way.”

Cheese as Economic Stimulus

Even without the visits from celebrities, Fisher thinks grilled cheese is a recession-proof plan. She even has evidence: When the stock market went down in 2007, sales for The Pop Shop went up.

“When you're feeling anxious, it's what you want,” she says. “You want the carbs, and you want that feeling of nostalgia.”

Matt Fish, owner of The Melt Bar and Grilled in Ohio, agrees. Fish, named one of 2010's most interesting people by Cleveland Magazine, opened his first grilled cheese-centric restaurant in 2006. By 2013, he had four locations, and he'll open a fifth this year.

“We actually grew through it,” he says. “We definitely rode the recession out.”

According to Fish, it's really comfort that's recession proof. "People have to eat no matter what," he says. "People go with what they're familiar with in hard times. You go back to what you're comfortable with; you don't take chances. And we're also known for comfort, which goes beyond the grilled cheese.”

But bigger, badder grilled cheese has helped put Melt on the map for its decadence as much as its comfortable familiarity. Melt has sandwiches that smash anything from lasagna to meatballs in a grilled cheese.

Slathered, Smothered and Branded

The restaurant also has an incredibly cheese-laden challenge. With 13 different cheeses, slaw and fries, The Melt Challenge involves eating about five pounds of cheese-covered food, and isn't for the faint of stomach.

The contest was featured on The Travel Channel’s “Man v. Food.” Between all Melt locations, about one customer daily takes on the challenge.

“We have about a 91 percent failure rate,” Fish says.

That's not the end of the grilled cheese fanaticism. Melt offers 25 percent off — for life — to customers devout enough to have the restaurant's logo tattooed on a body part.

Since 2009, almost 500 people have been branded. Talk about the power of grilled cheese.

“Every day is National Grilled Cheese Day for us,” Fish said. “It's what we do.”