In a large bowl, mix the Masa Harina and salt. Gradually pour in the warm water and use your hands to form a soft, pliable dough. If it's too dry and cracks, add a tablespoon of water. If it's too sticky, add a little flour.
The dough should be very soft to be easily shaped. Cover with a damp cloth and let stand for 10 minutes.
Grease your hands with a little oil or moisten them with water. Break off a golf-ball-sized piece of masa.
Flatten the ball into a disk and use your thumb to create a deep indentation, like a small bowl.
Place 1-2 tablespoons of filling (e.g., the mixture of cheese, beans, and chicharrón) into the indentation.
Carefully seal the meat by pinching the edges of the flour over the filling, forming a ball again. Remove any excess masa at the top.
Gently flatten the pupusa back into a disk about 1 cm thick, being careful not to let the filling burst out.
Heat a cast-iron griddle (comal) over medium-high heat (without oil or with minimal oil).
Cook the pupusas for 4-5 minutes on each side. When done, they should be golden brown and slightly puffed with melted cheese, which may seep out in places, creating crispy edges.
Serve the Pupusas immediately, hot, with a generous amount of cold Curtido (cabbage salad) and drizzled with Salsa Roja (tomato sauce).