Combine the masa harina, warm water and salt, mixing thoroughly. Let mixture stand for five minutes.
Knead with your hands for about 3 minutes, moistening your hands with water as you work.
Form 8 small balls with the dough. Place each ball between 2 plastic bags and with a flat pot cover, flatten to ½ inch.
Remove the top plastic of 4 tortillas, sprinkle the cheddar cheese, queso fresco and mozarella cheese over the bottom of each tortilla. Add the refried beans evenly on top of the cheese, then top with chicharron. Top with another tortilla. Press the borders to close the pupusa so the filling does not come out.
Add the oil to a nonstick pan over medium heat. Place the pupusas in the pan, and cook about 3 minutes on each side, until a crust forms or they are golden brown. Serve warm with curtido.
To make the curtidio:
Place all the ingredients in a bowl an mix well. Place in the fridge until ready to serve.