Fry 1 cup chopped onion in a skillet with 2 tablespoon olive oil for 2 minutes, stirring occasionally. Add 3 garlic cloves and stir a few more minutes, until softened and lightly browned. Be careful not to burn it.
Add 2 teaspoons tomato paste, 1 tablespoon chipotle chiles in adobo, 1 can diced tomatoes, ¼ teaspoon chili seasoning powder and 1 cup chicken broth. Stir to mix, bring to a boil over medium-high heat and simmer for 5 minutes on low heat.
In a large saucepan or pot, fry 1 ½ pounds chicken pieces in the remaining 2 tablespoons olive oil, seasoning with ½ teaspoon salt and ¼ teaspoon black pepper, or to taste. It will take 10-12 minutes for both sides.
Add the tomato sauce to the chicken (don’t wash the skillet) and the remaining 1 cup chicken broth. Simmer over low heat for 30 to 40 minutes, or until the chicken is soft enough to shred easily.
Separate the chicken into strands by pulling apart the meat with two forks.
Process the remaining sauce in a regular blender or with the immersion blender (my choice) until smooth.
Put the shredded chicken back in the skillet, add a little sauce and heat for about 5 minutes over low heat, stirring.
Let the mixture cool a little and serve on 8 tortillas with toppings of choice. See notes below.