Step 1Place the dried peppers in a bowl and cover with hot water. Weight the peppers down and set aside for several minutes to give the peppers time to soften.
Step 2Meanwhile, wrap the onion in aluminum foil and grill over a hot comal or griddle until the onion is charred and softened, 10 to 15 minutes. Remove from heat.
Step 3Over the comal or griddle, toast the garlic cloves, cinnamon, whole cloves, peppercorns, cumin seeds, oregano, thyme and bay leaves, tossing or stirring until the ingredients are aromatic, 2 to 3 minutes. Be careful not to burn. Remove from heat and set aside.
Step 4Combine the tomatoes and water in a saucepan. Bring the water to a boil and cook until the tomatoes are very soft, 15 to 20 minutes. Remove from heat and set aside to cool.
Step 5Combine all of the above ingredients in a blender and blend until everything is fully pureed, several minutes. Taste and season with 1 tablespoon salt, or as desired. Combine the marinade and chicken in a nonreactive container, cover and marinate overnight.
Step 6Heat the oven to 300 degrees. Transfer the chicken to a large baking dish and cover with the avocado leaves. Cover the dish with aluminum foil and roast until the chicken is tender, 60 to 75 minutes.