Step 1

Heat a large skillet over medium. Once warm, add the oil and onion. Sauté for 4 minutes or until tender, stirring occasionally. Add in the garlic and cook for 30 seconds more. Stir in the chipotles, oregano, and cumin, and toast for 1 minute. Add in the tomatoes, stock, and salt. Bring to a simmer, and cook for 7 minutes.

Step 2

Place the tomato mixture in a high-powered or regular blender, and blend until smooth.

Step 3

Return the blended sauce to the pan over low heat. Add the chicken, and cook for 5 minutes. Taste and add more salt if necessary.

Step 4

Prepare the garnishes. To assemble, top the tortillas with the chicken and garnish with the avocado slices, cilantro, red onion, and cotija. Serve with a lime wedge for squeezing.