1Place chicken breasts in a pot with one whole onion, two bay leaves, and a tablespoon of chicken bouillon. Cover with water and boil until cooked through.
2Add three roma tomatoes to the boiling water to soften.
3Remove softened tomatoes and place them in a blender along with the chipotle peppers, another tablespoon of chicken bouillon, and a splash of chicken stock. Blend until smooth.
4Sauté sliced onion in a pan with olive oil.
5Pour the blended sauce into the pan with the sautéed onions.
6Shred the cooked chicken using a stand mixer.
7Add the shredded chicken to the sauce and stir to combine.
8Assemble the tostadas by spreading crema on a tostada shell, topping with a portion of the chicken tinga, and garnishing with shredded lettuce.