In a large bowl, mix the masa harina, water and salt. Add more water if necessary, 1 tablespoon at a time, until smooth and forms a stiff dough. Cover with plastic wrap, and allow dough to rest for 15 minutes.
With lightly oiled hands, divide dough into about 6 pieces. Form each piece into a disk, about 1/4 inch thick. Spread 1 tablespoon of beans and cheese in the center, cover with another disk and seal closed.
Lightly oil the surface of a cast-iron skillet set heat to medium heat. Cook the pupusas for 5 minutes on each side or until golden brown and crispy.