Ingredients:

  Amount  Measure       Ingredient -- Preparation Method
--------  ------------  --------------------------------
     1/2  pound         roma tomatoes
   1      medium        white onion -- chopped
     3/4  pound         tomatillos -- husked/washed
   4                    garlic cloves -- crushed
  40                    arbol chiles -- (1 cup)
   2      cups          water
     1/2  bunch         cilantro -- leaves
   1      teaspoon      salt -- only, roughly

Preparation:

Preheat the broiler. Place the tomatoes and tomatillos on a baking sheet. Broil, turning occasionally, until charred all over, 10 to 12 minutes. Transfer to a saucepan along with the remaining ingredients. Bring to a boil and cook until the 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. SOURCE: Mesa Mexicana, Mary Sue Milliken and Susan Feniger with Helena Siegel.