Chicken tinga is becoming more common north of the Border, but most of the versions you encounter will be tomato-based and a bit lackluster. By adding tomatillos to the sauce you'll get a tartness that combines flawlessly with the chipotles in adobo. The result is an authentic tinga taco that you'll come back to time and time again. You can also make tinga tacos (or tostadas) with pork.
If you don't have fresh tomatillos, you can use canned tomatillos and get a similar result. Don't worry about roasting the tomatillos if you are using canned, as it doesn't have the same effect on tomatillos that have been resting in liquid.
If you are cooking the chicken, cover 2 chicken breasts with water in a saucepan. Bring to a boil and simmer for 20-25 minutes or until the chicken is no longer pink inside.
Roughly chop 1/2 an onion and peel 2 garlic cloves.
Cook the onion and whole garlic cloves in a dollop of oil on medium heat until the onion is just starting to brown.
You can pan roast the tomatoes and tomatillos if you want, but I usually just use the oven. Roast the tomatoes in the oven at 400ºF for 15-20 minutes. Add the tomatillos to the tomatoes about halfway through the roasting period as they need less time to roast than the tomatoes.
Add the onion, garlic, tomatoes and tomatillos to a blender along with 3 chipotles in adobo (I usually scrape out the veins and seeds of the chipotles).
Pulse blend until combined well.
Simmer this sauce in a dollop of oil on medium heat.
Add 1/2 cup stock, 1/8 teaspoon oregano, a dash of cumin, and a pinch of salt to the sauce.
Let reduce for 5-10 minutes.
Add the shredded chicken and combine well with the sauce, bringing everything to a uniform temperature.
Give a final taste before serving and add more adobo sauce if you want more heat, and add salt to taste.
Garnish with finely chopped avocado bits, cheese, diced onion, and a sprinkle of lime juice.
If crisping up the tortillas, add the tinga mixture to a tortilla and cook on a skillet or comal over medium-high heat until brown spots appear on the underside (2-3 minutes).