Place the chicken into a large pot with all of the ingredients listed for chicken. Cover with water. Bring to a boil and reduce heat to a simmer and cook for 1 hour. During the last 10 minutes, add the guajillo peppers to the pot with chicken. Remove from heat and let chicken and peppers cool in broth.
Place the tomatoes under the broiler or cook on the comal (at medium heat) until skins begin to blackenand blister. Turn as needed. This takes about 20-25 minutes. Remove from heat.
To the blender, add the roasted tomatoes, softened guajillo peppers in the chicken stock/broth, 1 cup of chicken stock, salt and pepper to taste. Blend on high until very smooth. Strain through a wire mesh strainer into a large pot with 2 tablespoons of oil heated to medium. Lower heat to a simmer, taste for salt and continue cooking for 7-9 minutes. You could add a pinch of Mexican oregano if you like.
Remove the chicken from the broth and shred. Set aside. In a large pot, heat 3-4 tablespoons of oil to medium heat. Add the onion strips and minced garlic. Season lightly with salt and pepper and saute for 5-7 minutes. Add the shredded chicken and the caldillo that has been simmering.
To the blender, add the chipotles in adobo. Add 1 cup of chicken stock. Blend on high until smooth. This is where you chicken becomes Tinga! In most traditional tinga recipes, there is plenty of chipotle added. But I like to gradually stir in the blended chipotle to my desired heat level.
Once you have added your chipotle to the tinga, stir well to combine. Taste for salt and pepper. Cook at a steady simmer, partially covered for an hour or until the sauce reduces by 3/4. The tinga should not have alot of broth, but should be more on the thick side for serving on tostadas.
Serve on tostadas. Garnish with lettuce, tomato, sliced onion, avocado, crema and queso fresco.