Once hot, add onion and taco seasoning and sauté for 5 minutes or until tender, stirring occasionally.
Stir in garlic and chipotles, and cook for 30 seconds.
Stir in the tomato sauce, broth, and chiles.
Bring to a simmer and cook for 5 minutes.
Stir in the chicken, then reduce heat to low and cook for 5 minutes.
Taste and add more salt if necessary.
Serving suggestion: place a tostada shell onto a plate, then top generously with a layer of refried beans. Top with cooked chicken mixture, then garnish with shredded lettuce, avocado, pickled red onions, cilantro, cheese, and a squeeze of lime.