Poach the chicken

• Bring water to a boil in a pot.
• Add half the onion, the garlic cloves, the bay leaf, the chicken bouillon and the chicken breasts.
• Cook until the chicken is fully cooked through, then remove and shred it.

Make the chipotle sauce

• Sauté the halved tomatoes with the chipotle peppers in adobo until the tomatoes soften.
• Add the water and let it simmer a couple of minutes, then turn off the heat.
• Blend everything with the black pepper, oregano and salt until smooth.

Cook the onion and sauce

• Sauté the remaining thinly sliced onion for a few minutes.
• Pour in the blended sauce and add a little more water.
• Let it simmer for a few minutes.

Add the chicken

• Add the shredded chicken to the simmering sauce.
• Mix everything together until the chicken is well coated.

Assemble the tostadas

• Place the tinga on a tostada.
• Top with lettuce, sour cream, avocado slices, queso fresco, lime juice and cilantro.