Heat the vegetable oil in a medium skillet over moderate heat. Cook the onions until soft, about 10 minutes. Transfer to a food processor fitted with the metal blade or a blender. Add the remaining ingredients and puree, in batches if you are using a blender, until smooth. Pass through a medium strainer, pressing with a spatula or wooden spoon to push through as much pulp as possible. Pour into a saucepan, and add the salt. Bring to a boil, reduce to a simmer and cook, uncovered, 20 minutes. Adjust the seasonings. Set aside and cool for table salsa or use warm for red rice or chilaquiles. Makes 1.5 quarts Store in the refrigerator 2-3 days or in the freezer for weeks. SOURCE: Mesa Mexican, Mary Sue Milliken and Susan Feniger with Helena Siegel.