Wipe off any dusty crevasses on the dried chilis. De-stem and de-seed the chilis, but don't worry about getting rid of every last seed.
Roast the chili pieces for 1-2 minutes in a 400F oven. Add them to a bowl and cover them with hot tap water. Let the chilis reconstitute for 20-30 minutes. If they float to the surface you can use a small bowl or plate to keep them submerged.
Roast 7-8 plum tomatoes in a 400F oven for 20-30 minutes.
Roughly chop 3 onions and peel 8 garlic cloves. Add a dollop of oil to a skillet on medium heat and saute the onions and whole garlic cloves until lightly browned.
You'll probably have to combine the ingredients in two batches. Add half of the tomatoes, chilis, onion mixture, and stock to a blender and combine well. Strain the sauce through a fine mesh sieve and discard the leftover skin and seed remnants. You might have to use the back of a spoon to push the sauce through the strainer.
Add a dollop of oil to a saucepan on medium heat. Add the enchilada sauce along with 1 tablespoon Mexican oregano, 1 teaspoon cumin, 2 teaspoons salt, and some freshly cracked black pepper. Simmer for 30-45 minutes or until it has reduced down to a velvety consistency. Take a final taste for seasoning.
Let the sauce cool and then portion it into 2-cup portions (or larger if you want to, you'll have about 10-12 cups total). I typically keep a portion in the fridge and the rest in the freezer. Mason jars and Ziplocs are both good options for storing in the freezer.
If you want to make a batch of chicken enchiladas, please see our Ancho Chicken Enchiladas post for details.