This sauce is a great condiment for Mexican food.
Wash chiles, pat dry and tear into long strips. Discard stems and seeds. Heat a large heavy skillet over medium heat. Toast chiles, turning once or twice, for 2 to 4 minutes.
Transfer chiles to a bowl. Add simmering water to cover and set aside until chiles have softened, about 20 minutes. Drain. Place softened chiles, 1/2 cup broth, 1/2 cup water, garlic and oregano in a blender; blend until very smooth. (If the mixture is too thick, add just enough water to give a smooth consistency.)
Heat oil in a medium saucepan over medium heat. Add the chile puree and sugar; reduce heat to low and simmer, stirring constantly, until sauce has darkened and thickened, about 10 minutes.
Remove sauce from heat and add the remaining 1/2 cup broth. Strain sauce through a fine sieve, pushing puree through sieve with a wooden spoon. Adjust seasonings with salt and more sugar if desired.
Make Ahead Tip: The sauce can be stored, covered, in the refrigerator for 1 week.