In a large mixing bowl, combine masa harina, warm water, and salt until it forms a soft dough.

Divide the dough into 16 equal parts and roll them into balls.

Flatten each ball with your palm and make an indentation in the center.

Fill the indentation with a spoonful of refried beans, a sprinkle of queso fresco, onion, and cilantro.

Pinch the edges of the dough together to seal the filling inside and reshape it into a ball.

Flatten the ball into a disk about 1/4 inch thick.

Heat a griddle or skillet over medium heat and brush it with vegetable oil.

Cook the pupusas for 2-3 minutes on each side or until they turn golden brown and slightly crispy.

Serve hot with a side of refried beans, curtido (a Salvadoran cabbage slaw), and salsa.