Cut off stem ends of tomatoes and roast them in a cast-iron skillet over high heat until their skins blacken. Transfer to a blender or food processor. Dry skillet and return to high heat. Add dried chiles and roast them, stirring, until they begin to release their aroma, about 3 minutes. Transfer chiles to a bowl and pour water over. Leave to soften and rehydrate, 10 to 15 minutes. Add softened chiles to tomatoes. Taste chile water; if it is not bitter, add 1 cup to food processor and reserve remaining water separately, or use plain water. Wipe out skillet, heat olive oil over high heat, add onion and saute over medium heat until browned, about 10 minutes. Transfer onions to food processor along with chipotle pepper with its sauce, garlic, cumin, oregano and salt to taste. Puree to a fine paste, stopping once or twice to scrape down sides of workbowl with a rubber spatula. If needed, add a little more water to processor.
In a large skillet heat vegetable oil until just smoking, add chile puree and heat until sizzling. Fry sauce, stirring continuously, 3 to 5 minutes. If sauce becomes too thick, thin it with a little more chile water.
Serve warm with tamales or grilled meats. Or cool and refrigerate for up to 1 week.
Recipe courtesy of David Rosengarten