Prepare the dough: In a large mixing bowl, combine the masa harina, salt, and warm water. Mix until the dough is smooth and pliable. It should have a consistency similar to playdough – not too wet or too dry. If it's crumbly, add a little more water; if it’s too sticky, sprinkle in more masa harina.
Shape the dough: Divide the dough into 8 equal portions. Roll each piece into a ball, then flatten it into a thick disc, about 4 inches in diameter.
Add the filling: Take one disc of dough and place about 1 tablespoon of cheese in the center. Add refried beans or pork if using. Carefully fold the edges of the dough over the filling, sealing it in the middle.
Flatten the pupusa: Once sealed, gently flatten the stuffed ball back into a disc, ensuring the filling is evenly distributed but not leaking out.
Cook the pupusas: Heat a skillet or griddle over medium-high heat. Lightly grease it with oil, then cook each pupusa for about 3-4 minutes per side, until golden brown and slightly crispy.
Serve with curtido: Once your pupusas are ready, serve them hot alongside the tangy curtido for a refreshing contrast.