In a medium skillet, heat the oil. Add onion and garlic and cook over moderate heat , until softened (3-5 minutes). Add the tomato, chipotles, cumin and nutmeg and cook, stirring for 3 minutes. Add the chicken and cook over moderate heat, stirring occasionally, until most of the liquid has evaporated (about 10 minutes). Season with salt and pepper.
When ready to serve, warm the tortillas, spread with the warm chicken filling and serve with sour cream (optional).
We tripled this recipe without any problem and froze the filling for later use. To serve, thaw the filling and heat through. The proportions listed above serve 12 as an appetizer. From Food & Wine Magazine, March 1998.