Place the ancho peppers and chicken stock into a small pot over medium-high heat. Bring up to a bubble and simmer until the peppers are very tender, about 15 minutes. Transfer the mixture to a food processor or blender and puree until smooth. Reserve.
Place a large skillet over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons. Add the red onion and garlic to the pan, and cook until tender, about 5 minutes.
While the veggies are cooking, remove the skin and meat from the chicken, and shred the meat.