Place the tomatillos, anaheim chili and pasilla chilies on a baking sheet and under the broiler, roast for 6-8 minutes, or until slightly charred.
While the tomatillos and chiles are roasting, heat the olive oil in a skillet over medium-high heat.
Add the onion and serrano peppers, saute 5-6 minutes, then add the garlic and saute for an additional minute. Remove from the heat and set aside.
Remove the chiles and tomatillos from the oven and cool slightly. Peel the skins from the chilies, and remove the seeds and stems. Place the chilies and tomatillos in a food processor or blender. Pulse for 3-5 seconds.
Add the onion, serrano, and garlic to the food processor with the remaining ingredients, except for the chicken. Blend until smooth.
Place the chicken in the bottom of a slow cooker and pour the sauce over the chicken. Cook on low for 6-8 hours, or high 3-4 hours.
Serve the chicken over quinoa or rice, or shred and serve in tortillas as tacos.