Step 1: Cut sandwich rolls in half and spread with butter. Toast until golden on a griddle or comal.
Step 2: Heat cochinita (pork) in a pan until warm, then set aside.
Step 3: On the bottom half of the toasted rolls, spread refried beans generously.
Step 4: Top beans with reheated pork, pickled onions, and avocado slices.
Step 5: Close the sandwich, press gently, and serve whole or sliced in half for easier handling.