To make the sauce, combine the boiled tomatoes, onion, canned chipotles, chicken broth, halved garlic cloves, and salt in a blender. Blend until smooth. Set aside.
To make the tinga, heat the olive oil on a 12" HexClad Hybrid Pan over high heat. Add the onion and minced garlic and sauté for 5 minutes. Add the chicken and stir to combine. Strain the tomato sauce through a fine-mesh strainer into the pan and stir to combine. Bring to a boil over high heat, lower the heat to medium-low, and simmer to reduce slightly, about 20 minutes.
Season the tinga with salt and black pepper to taste.
To serve, place a tostada on a plate. Spread with about 3 tablespoons of refried beans, then top with 1/3 to 1/2 cup of the tinga. Top with shredded lettuce, then avocado slices, crumbled cheese, and crema and hot sauce, if desired. Serve with lime wedges.