From the get-go, it has a primal ooze that shifts the body into comfort-zone overdrive.

At its essence are two simple ingredients: a carb crunch followed by a liquid blanket of calcium. And now the humble grilled cheese sandwich is evolving from home-style to haute cuisine.

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"I always think of [Venice's] Harry's Bar sandwiches: tiny little rectangles oozing a cheesy filling," says Marlena Spieler, whose "Grilled Cheese: 50 Recipes to Make You Melt" will be published by Chronicle Books in the fall. "Or pain levain (classic French country bread) filled with Beaufort cheese and mushroom sauce, and then I think about good grilled cheddar on whole-grain bread with a layer of tomato or onion inside. All are so good, so different, and yet the same basic thing: a grilled cheese sandwich."

At Belly General Store in Virginia-Highland, deputy storekeeper and cheese expert Raymond Hook offers a three-cheese panino made with award-winning fresh goat cheese, Clayburne cheddar and Green Hill, a cow's-milk double cream cheese from artisanal cheesemaker Sweet Grass Dairy.

Hook deems the ultimate grilled cheese one that "showcases the quality of the cheese." Belly also features a homemade mozzarella, tomato and basil panino, which Hook describes as "reaching a balance between the flavors and creaminess."

Rachael Ray, host of the Food Network's "30 Minute Meals," says the endless creativity in building on the theme works "because there are breads and cheeses that appeal to everyone. The grilled cheese can take on any personality."

On an episode of "Meals," Ray returned to the roots of the grilled cheese genre with a recipe for croque monsieur, a classic French cafe sandwich.

"That's my take on French grilled cheese and ham sammies," says the affable Ray.

The croque monsieur is a "grilled ham and cheese sandwich that is dipped into beaten egg before being sautéed in butter," according to Sharon Tyler Herbst's "Food Lover's Companion" (Barron's).

(In France, croque madame is similar to the male counterpart but with "the addition of a fried egg," writes Herbst. "In Britain and America, a croque madame simply substitutes sliced chicken for the ham, with no sign of an egg.")

There's a largely American cousin that deserves mention in a discussion of grilled cheese sandwiches: monte cristo. This layered treat consists of slices of cooked poultry, cheese (usually Swiss) and sometimes baked ham. The sandwich is dipped in egg and grilled until golden. But it's difficult to give anyone full credit for the grilled cheese category in cookbooks.

"Grilled cheese sandwiches really capture people's imagination," says cookbook author Spieler. "Almost everyone has the simple grilled cheese sandwich they grew up with, and the idea of a few new possibilities is exciting."

In developing recipes for her book, Spieler turned her attention to the innovative fillings — in essence thinking far outside the ubiquitous cellophane wrapper of processed cheeses.

"Cheese is really a great arena of learning," she says. "So many different types, with flavors and aromas and textures. Heat both brings out the best in some cheeses, and yet destroys the subtlety of others. There are hundreds of cheeses and myriad breads available."

But are there ingredient boundaries to great grilled cheese sandwiches? You bet.

"Some cheeses don't melt well — especially soft and mild cheeses," says Spieler. "They melt into nothingness, then run out of the bread, and you're left with a sodden, heavy thing."

But don't discount all soft and mild products, she says: "The trick was to toast the bread first, and then warm the cheese under the broiler.

"Goat cheese and feta cheeses were a delightful surprise, though as they don't melt, I found that by combining them with a layer of cheese that does melt, the sandwich had both the tang of fresh cheese and the melty, unctuous pleasure of Jack, fontina, mozzarella, etc."

By definition, cheese should dominate in a grilled cheese sandwich, but there are a variety of ingredients that can enhance the base element.

For Spieler, that could include sautéed chiles, chicken, browned eggplant, spinach and even anchovies. In testing recipes for the book, she discovered that stronger flavors were the most successful.

"The subtle ingredients tended to get lost," she says.

When choosing breads, baked goods with "herbs such as rosemary, dill, rye seeds, olives — even raisins — were quite delectable paired with melting cheese," says Spieler.

To achieve a great grilled sandwich, Spieler says, the main thing is not putting too much fat in the pan. "Lightly buttering the bread or brushing it with olive oil was the way to go," she says. "The bread will taste buttery or olive oily, and very crisp.

"And, of course, pressing is often a great way to crisp up the edges of sandwiches."

No matter how it's prepared, the most successful grilled cheese sandwiches are made by cooks who don't lose sight of the concept.

"At the soul of the sandwich — even the ones that require a sauce to be simmered, or other ingredients to be prepared — is the play of melted cheese on toasted bread," says Spieler.

As with the Thanksgiving stuffing/dressing debate, where you live may determine how you refer to melted cheese locked between two slabs of bread.

"Toasted cheese is a term I've heard in Britain," says Spieler. "Melted cheese, as well. Never grilled cheese. As far as I can see, though, toasted cheese seems to have been a Scottish and possibly Yorkshire term. But they both seem to be the same to me, and I have no problem with it being used interchangeably."

— Jeanne Besser contributed to this article.