In a large mixing bowl, combine the masa harina and warm water. Mix well until the dough comes together.
Add the shredded mozzarella cheese, shredded cheddar cheese, finely chopped onion, and salt to the mixing bowl. Mix until all the ingredients are evenly distributed.
Divide the dough into 8 equal portions and roll them into balls.
Using your hands, flatten each ball into a disc shape, about 1/4 inch thick.
Heat a non-stick pan or griddle over medium heat. Lightly brush the surface with vegetable oil.
Cook the pupusas on the heated surface for about 3-4 minutes on each side, or until golden brown and cooked through.
Serve hot with your choice of toppings, such as curtido (a cabbage slaw) or salsa.
Notes
Tip and Tricks
Make sure to use warm water when mixing the masa harina to help the dough come together.If the dough is too dry, add a little bit more warm water, one tablespoon at a time,until it reaches the desired consistency. Use a non-stick pan or griddle to prevent the pupusas from sticking.