Heat a comal or a cast iron skillet over medium heat until a drop of water sizzles on contact. Remove the stems and seeds from the chiles and wipe them clean with a dry cloth. Place them on the comal and toast them, just until their aroma is released, 30 to 60 seconds. Be careful not to burn them. Place the chiles in a bowl and cover them with boiling water. Let them soak until softened, about 10 minutes. Drain the chiles and discard the liquid. Place the chiles, oregano, garlic, and the 2 cups of water in a blender and blend to a smooth puree. Add more water if the sauce is too thick. Place the oil in a medium size saucepan. Heat the oil over medium high heat until it is rippling. Add the flour and cook, stirring constantly, until golden. Add the chile puree and salt to the pan and reduce the heat to low. It may splatter as you pour it in, so be careful. Cook, stirring often, until the raw taste is gone and the flavor of the chiles has mellowed, about 10 minutes. Finish the sauce with the sugar and vinegar. Store, tightly covered, in the refrigerator up to a week or indefinitely in the freezer.
c.1997, M.S. Milliken & S. Feniger, all rights reserved