I am lucky enough to have a fabulous tortillaria near my home that sells fresh organic masa. I make everything from challah bread to corn dogs with it. But if I can't make it over there, I make my own masa from dried organic corn and "cal." Blows masa harina out of the water. These pupusas aren't made quite like a Salvadorian grandma would, but they're delicious. You can fill them with near about anything. They freeze well too (heat them thoroughly in an oven before serving). Bring these to a party and, trust me, there will be no leftovers. —cyndin
Start with fresh masa, which is the dough used for tortillas. The best is to get it ground locally...you may have luck if there is a restaurant or shop near you that makes its own tortillas. Some Mexican/Latin markets will also have it. You want the kind for tortillas (plain corn, lime, and perhaps salt), not the kind for tamales (has lard added). You can also make your own masa from dried corn and lime (calcium carbonate). In a pinch you can use the powdered stuff, masa harina. It won't hold together as well. Do not try this recipe with pre-made tortillas.
Thinly slice onions and mushrooms and saute in olive oil with a bit of salt until the onions are caramelized and the mushrooms have given up their water. Set aside.
Heat a cast iron or other flat pan (something you'd make pancakes in) to medium heat. If it's well seasoned, you don't need any oil. Otherwise, you can use a very light coating.
Create a ball of masa approximately the size of a golf or ping pong ball. Slap it between your hands, or push with your fingers, until you have something that looks like a thick tortilla. Place on the griddle. Note: for a party you might want to make the pupusas smaller.
In the middle of the dough, put a dollop of pesto and a forkful of onions and mushrooms. Don't overstuff.
Create another ball of masa and flatten to a thick tortilla. Place on top of the filled one on the griddle. Crimp edges so it holds together.
Cook until the masa begins to brown, then turn. You don't want it cooked until hard but you do want the dough not to be raw. You may need to push down on the pupusas a bit to cook the edges.
Serve hot or warm on platters with a mayonnaise avocado dipping sauce (just mash avocados, add salt, lemon, and mayonnaise).