Preheat broiler. Place tomatoes and tomatillos on a baking sheet. Broil, turning occasionally until charred all over, 10 to 12 minutes. Transfer to a saucepan. Add the remaining ingredients.
Bring to a boil and cook until onions are soft, 12 to 15 minutes. Transfer to a food processor or blender. Puree and then strain. Serve at room temperature or slightly chilled. Arbol Salsa can be stored in the refrigerator 3 to 5 days, or frozen for weeks.
c.1997, M.S. Milliken & S. Feniger, all rights reserved