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 1/2 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.