Cook the Chicken:
In a large pot or Dutch oven, add chicken, onion, garlic, bay leaf, and salt. Cover with water (plus 1 inch). Bring to a boil, then simmer, covered, for 15–20 minutes until chicken reaches 165˚F.
Remove chicken and reserve 1 cup broth. Let chicken cool slightly, then shred.

Roast the Vegetables:
Place tomatoes and onion halves on a greased baking sheet. Broil for 10 minutes, flipping halfway through, until charred.

Make the Sauce:
Blend roasted tomatoes, onion, chipotle peppers, garlic, oregano, salt, cumin, and ½ cup reserved broth until smooth.

Simmer the Sauce:
Heat olive oil in a large pan over medium-high heat. Add sauce, cover, and cook for 10 minutes, stirring occasionally.

Combine Chicken and Sauce:
Stir in shredded chicken and cook for another 10 minutes. Add more broth if sauce is too thick.

Serve:
Spoon into tortillas and top with cotija cheese, diced onion, cilantro, and avocado.