Heat butter in a medium skillet over MEDIUM heat. Add chopped onion and sauté until onion becomes translucent and begins to turned light golden brown. Add minced garlic and saute 30 seconds more.
Put sauteed onion and garlic mixture along with all the remaining ingredients into a slow cooker.Cook on LOW for 4-6 hours [or HIGH for 2 hours].Sauce is ready to use immediately.
OPTIONAL STEP: If you prefer your enchilada sauce smooth, use an immersion blender to puree it right in the slow cooker.
Notes
Chili Peppers Substitution: Omit all 3 dried ground peppers, and add 2 tablespoons of regular chili powder. The flavor will be slightly less complex, but delicious just the same.This recipe freezes well, but it can also be canned in a pressure canner. Use 10 lbs. for 50 minutes for pints and 60 minutes for quarts.