Combine the corn flour, salt, and water in a mixing bowl. Work it together until a dough ball forms. You want it to be soft, smooth, and moist, so add a bit more water if necessary, a tablespoon at a time.
3 Cups Corn Flour, 2 Cups Warm Water, 1 Tsp Salt
Cover the bowl and let it sit for about 15 minutes, to allow the moisture to soften the corn flour.
In another bowl, mix together the shredded mozz and the crumbled queso. If you want to add other fillings, add it to this mixture. Set this aside.
1 Cup Shredded Mozzarella, ½ Cup Queso Fresco
Divide your dough ball into 8-10 portions.
Working with one dough portion at time, pat them flat into large discs. Add about 1-2 tbsp of your cheese filling to the center of the disc. Fold in the edges and pinch them together to seal them closed. Use a bit of water to dampen your hands and make the dough sticky enough to seal nicely.
Roll your sealed pupusa into a ball, then flatten it into a quarter-inch thick disc and set it aside. Repeat until all of your pupusas are prepared.
Heat your cooking pan over medium heat and lightly grease it with some oil. Cook the pupusas for 5-6 minutes per side, until they're golden and crisp. If you're working in batches, take the completed pupusas and put them on a wire rack in a warm (not hot) oven to keep warm while you cook the whole batch.
serve with your choice of toppings, like coleslaw, salsa, refried beans, or anything else you like.