In a mixing bowl, combine the masa harina and salt. 

Add the water and mix with a spoon until the mixture thickens. 

Knead the dough with your hands until it becomes smooth, then let it rest for 5 minutes. 

Divide the dough in 14 balls. 

With your hands, make a pocket in the middle of each ball. Place 1 tsp. of refried beans and 1 tbsp. of shredded sheese into the pocket and cover with the edges of the dough. Reform the ball and flatten into a thick disk shape. 

Place oil in a non-stick pan and heat over medium-heat. 

Once hot, add a pupusa to the pan and cook for 3 minutes on each side, or until sides begin to brown. 

Remove from pan and place on plate.