In a large mixing bowl, gradually stir water into masa harina until dough forms a ball that can be handled.

Into another bowl, empty the refried beans. Place the grated cheese into a separate bowl.

Divide dough into about 25 pieces. Roll each into a ball and flatten between the palms of your hands to about 1/2-inch thick. Put a spoonful of beans and a small handful of cheese into the center of each pupusa. Flatten again with the filling inside.

Heat a heavy, wide-bottomed or flat skillet until hot. Brush with oil and cook pupusas on each side 4-5 minutes until a pale golden-brown. The outside should be firm.