Mix masa harina and salt together in medium bowl. Add boiling water and vegetable oil and mix with rubber spatula until soft dough forms. Cover dough and let rest for 20 minutes.

While dough rests, lay parchment paper out for dough to rest on. Mix cheeses, form into small balls with the beans.

Knead dough in bowl for 15 to 20 seconds. Test dough's hydration by flattening golf ball–size piece. If cracks form around edges, run your hands under warm tap water and fold dough together until soft and slightly tacky. Transfer dough to counter, shape into large ball, and divide into equal portions. Using damp hands, roll 1 dough piece into ball and place on empty half of prepared sheet. Cover with damp towel. Repeat with remaining dough pieces.

Fold parchment paper in half and place in between two large plates, bottoms together. Place the dough balls in the parchment paper and squish between the two plates. Aim for ¼" to ½" thick.

Wrap the dough sheets around the cheese/bean filling and pinch the ends together. Roll into a ball and re-flatten the same way.

Cook pupusas on dry or lightly oiled skillet. Wait until the edge of the pupusa has a colour change midway through, then turn over and cook until the edge is evenly coloured. Top and bottom should have brown spots. If the edge cracks and cheese spills out, it'll only taste better.

Optionally serve with curtido and salsa.