The best thing since sliced bread is most definitely "The Mousetrap": sliced sourdough stuffed with sharp cheddar and havarti. At least that's what The American Grilled Cheese Kitchen's co-owner Nate Pollack will tell you about his 10-month-old restaurant's best-seller. He and his wife and business partner Heidi Gibson sell about 100 Mousetraps daily, most of them with added Applewood-smoked bacon and tomato. "The most surprising thing about operating a grilled cheese restaurant is how much regular business we get," says Pollack. "We have people who eat here every day for breakfast and lunch."

Who knew? There's a market— a big one—for "adult" grilled cheese. Pollack tipped us off that April just so happens to be grilled cheese month. And as any self-respecting grilled cheese restaurant should, The American is celebrating.

They'll bring back a special grilled cheese sandwich from months past each week throuhgout April. This week, look out for a "Hawaiian" with roasted pineapple, smoked ham, caramelized onions, fresh mozzarella, fontina and red chile flakes on Pinkie's levain.

You can also indulge at Cowgirl Creamery's six-month-old spinoff Sidekick, where co-supervisor Sarah Fine says they've lately been selling roughly 60 seasonal grilled cheese sandwiches per day. Today's offering, for example, is a Laura Werlin sandwich made with double cheddar, tomato jam and cheddar butter brushed on the outside before grilling. Another favorite Sidekick menu staple, the "toastie" is an open-faced grilled cheese layered with sharp Cabot cheese, caramelized onions and maple mustard with pickled vegetables served on the side.

Rounding out the representation, the Mission will get a grilled cheese contender later this month in Mission Cheese. Owner Sarah Dvorak will focuse on all domestic products and a menu of nibbles with several pressed cheese sandwiches. Dvorak will also have a seven-seat bar, stocked with beer and wine on tap and a cheesemonger to walk you through the finer points of what's beneath the bread.

Since San Francisco's fancier grilled cheeses may have anything from mushroom duxelle to gruyere inside, they can come across as costly for something with such humble origins. Pricetags at Sidekick range seasonally from about $7 to $9, depending on the cheeses and other ingredients used. At The American, sandwiches hover around eight bucks.

"Sometimes we'll get a customer who says 'I can make grilled cheese at home, so why would I pay for this'," says Pollack. "Well, we spend from four hours to two days on the mustards, caramelized vegetables, purees and relishes that go with our grilled cheeses; so sure, our customers can make these things themselves, but it's going to take them a good 48 hours."