For each pupusa, with lightly oiled hands, form the “masa” or dough into balls about 2 inches in diameter. Using your thumb, make an indentation into one of the balls, forming a small cup. Fill the cup with about 1 tablespoon of the zucchini, onion, and cheese mixture. Making sure that the filling does not leak, pat the masa together back and forth between your hands to create a flat, pancake-like dough, roughly ¼-inch thick and 4 inches in diameter.