Make the Sauce: In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add 1 cup of chopped sweet onion and sauté for about 4 minutes until softened. Then, add 2 cloves of minced garlic, 1-2 chopped chipotle peppers, 1 teaspoon of dried oregano, and 1/2 teaspoon of ground cumin. Stir everything together for about one minute, allowing the spices to bloom. Next, add 3/4 cup of crushed fire-roasted tomatoes, 1/4 cup of chicken stock, and 1/2 teaspoon of salt. Bring to a gentle simmer and let it cook for around 7 minutes.
Blend: Carefully transfer the sauce mixture to a blender. Blend until smooth and creamy. This step is crucial for achieving the signature velvety sauce that coats the chicken.
Cook Chicken: Return the blended sauce to the skillet over medium heat. Add 3 cups of shredded cooked chicken to the pan. Stir well to combine the chicken with the sauce and let it simmer for an additional 5 minutes. Taste and adjust the salt as needed.
Assemble: To assemble your tacos, warm 10 (6-inch) corn tortillas either on a skillet or directly over a gas flame until slightly charred. Fill each tortilla with the chicken tinga mixture, then top with sliced avocado, chopped cilantro, diced red onion, and crumbled cotija cheese. Serve with lime wedges on the side to squeeze over your tacos before devouring!