Cook the chicken breasts in boiling water or broth until tender and fully cooked. Once cooled, shred the chicken using two forks and set aside.
In a blender, combine the fire-roasted tomatoes, chipotle peppers in adobo sauce, garlic, oregano, and chicken broth. Blend until completely smooth.
Heat the olive oil in a large skillet over medium heat. Add the sliced yellow onion and sauté for 5 to 7 minutes until translucent and slightly caramelized.
Pour the blended tomato and chipotle sauce into the skillet with the onions. Bring to a simmer and cook for 5 minutes.
Stir the shredded chicken into the sauce, ensuring the meat is fully coated. Simmer for an additional 10 minutes until the sauce thickens and the chicken absorbs the flavors.
Warm the refried beans in a small saucepan or microwave.
Assemble the tostadas by spreading an even layer of refried beans on each crispy corn shell.
Top the beans with a generous scoop of the warm chicken tinga.
Garnish evenly with shredded lettuce, pickled red onions, crumbled cotija cheese, and avocado slices. Serve immediately with fresh lime wedges.