This dish is a mainstay at Sarah Esperanza's fantastic Central American grocery store, El Izalco, which is located at 696 Sargent Ave. The following recipe is from Sarah (minus the Lizano sauce), whom you can book cooking classes with at El Izalco.
(a traditional El Salvadorian dish which is essentially a stuffed corn flatbread)
one can of black beans
half an onion (chopped)
ancho chili powder
For the pupusa shell:
Mix about 3 cups of masa powder with two cups of warm water. You then want to work it into a firm, yet moist dough.
Once dough is done, start pulling out balls between the size of a golf ball and a tennis ball then flatten by working back and forth in your hand. You want to make disks about 3 inches in diameter, with the dough not being too thin.
For the filling, cook black beans with onion, garlic, and ancho chili powder until onions are soft. Once cooked, blend the bean mix with a good spoonful of Costa Rican Lizano sauce until it becomes a fairly thick puree.
Pour the puree into a bowl, then scoop out about two Tbsp of paste onto your individual discs of dough. Sprinkle on a soft grated cheese like the queso fresco, or mozarella, then fold up the edges of the dough to reform a ball with all the filling inside. Flatten that ball back to a disk size that is slightly thicker than a pita (about a 1/3 inch thick). Grill the pupusas on a hot pan until each side gets a bit brown -- this shouldn't take more than a minute a side.
Once cooked, top them with curtido
-- that spicy Salvadorian cabbage whose acidic heat cuts right through the dough -- and serve. I'd recommend buying the curtido from El Izalco, I've tried to replicate that slaw at home but still haven't made it nearly as well as Sarah does.