Cut the pork into medium pieces and place in a pot. Cover with water, add 4 peppercorns, a bay leaf, 1/4 onion, 1 garlic clove, and salt. Cook for 1 1/2 hours. Let cool, then shred the meat. Cook the tomatoes. Blend the beans until smooth—you can use them as is on your tostada, or fry them briefly in oil or lard.

Slice the lettuce. Separately, slice the onion and place it in water with a splash of vinegar, 2 cloves, a pinch of dried oregano, 2 peppercorns, a pinch of salt, and a pinch of sugar to marinate. Once the tomatoes are cooked, peel them and blend with 1 garlic clove, a little onion, some of the cooking water, and salt. After blending, add 1/2 teaspoon dried oregano, mix, and set aside.