Place the ancho chili 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 two 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.
Add the sherry or sherry vinegar to the pan and cook for a minute to burn off the alcohol. Add the oregano, cumin, coriander, shredded chicken and ancho puree to the skillet and cook (add a splash of chicken stock if the mixture is too thick) until heated through, 3-4 minutes.
Portion the meat mixture into tortillas and serve.