My husband's grandmother would never speak to me again if she knew I was sharing this recipe. But she doesn't have a computer, so...
Toast 2 unpeeled cloves of garlic over medium heat in a dry skillet. Let cool, then peel and toss the cloves in a blender.
Toast 4 dried guajillo chilies in the same dry skillet over medium-high heat, about 15 seconds on a side, pressing down with a spatula. You'll see the pepper skin get shinier, and a little redder. After each pepper is toasted, transfer to a pot of almost-boiling water. Cover the pot and let sit for 20-30 minutes.
Remove the peppers from the water, drain, stem and seed. Add the pepper flesh to the blender, along with an 8-oz can of tomato sauce. Add a few tablespoons of the soaking water to rinse out the can, and add to the blender. (But taste the water first - if it's bitter, just use un-chilied water.) Buzz until pureed. Stir in a little Mexican oregano, salt to taste, and more water if you want the sauce thinner.
This is good on everything - meat, eggs, fish, potatoes, chips, beans, etc. (Though probably not good on marshmallows...)