Saute the onion in olive oil, then add the garlic and cook for 30 seconds. Stir in the chipotles, oregano, and cumin and cook for 1 minute. Add the tomatoes and stock, cook for 7 minutes. Season to taste. Puree the mixture in a blender until smooth.
Return the sauce to the pan over low heat, add the chicken, and heat through.
Plate the mixture on the tortillas, garnish with the avocado, cilantro, red onion, lime and cheese, and serve.