Oven roast the chiles until slightly darkened and fragrant, turning once. Since the peppers are dry, this will not take long, so keep an eye (and nose) on them at all times. Let cool until you can handle them.
Cut off the stem ends of the chiles and pour out the dried seeds. Cut or tear the chiles into pieces and set aside in a small bowl.
Saute the chopped onions and garlic in the vegetable oil until translucent and just starting to color. Add the salt and cumin to taste.
Meanwhile, heat the beer or stock until just below a boil and pour over the chiles. Let soak until softened.
Add the chiles and liquid along with the onion mixture to your blender. Pulse until smooth. Be careful when blending hot mixtures.
Return the puree to a sauce pan, add 2 cups of water and bring to a boil. Reduce heat to medium-low and simmer until thick and well-reduced, about an hour.