Place chile peppers on a cookie sheet and roast in the oven until slightly toasted. (Note: When you can smell them, they're done.)
Remove from the oven and slit them open. Remove the seeds and veins, then put them in a bowl with boiling water to cover. Allow them to sit for 30 minutes, then remove them from the water, reserving the water. Scrape the pulp from the insides of the chile peppers and discard the skins. (Note: The skins will give the mixture a bitter flavor.) Return the pulp to the water and set aside for later.
In a large saucepan over medium heat, saute the garlic and onion in the butter or margarine for 5 minutes, or until onion is tender. Add the flour and mix well. Add the stock, stirring until thickened, and then add the reserved chile pulp mixture. Finally, add the oregano and the cumin.
In a blender or food processor, puree the mixture until smooth. Season with salt to taste.