In a large bowl, add the corn masa flour. Gradually pour in the water, about 1 cup at a time, kneading as you go. The dough should be soft, smooth, pliable, and moist—but not overly sticky. You’ll likely need about 3 cups of water for 4 cups of Maseca. If the dough feels too dry, add a little more water; if it’s too wet, sprinkle in more masa.
In a separate bowl, mix together the shredded mozzarella, Monterrey Jack, and refried beans until well combined. This will be your filling.
Scoop a portion of the masa slightly larger than a golf ball and flatten it into a thick disc. Add about 2 tablespoons of the filling to the center, then carefully fold the edges over to enclose the filling and form a ball again. Flatten it gently between your palms to form a thick, round pupusa.
Lightly wet or oil your hands as needed to keep the dough from sticking while shaping.
Place the pupusa onto a hot comal, griddle, or skillet over medium heat. Cook for 3–5 minutes per side, or until golden brown with a slightly crisp exterior. Repeat with the remaining dough and filling.
Pupusas are best served hot with curtido and a mild red salsa on the side.
Store any leftovers in the fridge for up to 3–4 days. Reheat on a hot skillet or comal for best results.