Cook all ingredients in a saucepan on medium high heat, stirring frequently so that beans do not stick to the bottom.
Once onions are cooked (around 10 minutes), remove from heat and using a potato masher, mash the beans, tomatoes and onion.
If there is still some water in the mixture and it is not a thick consistency place back on minimum heat and simmer until bean mixture is thick, stirring often.
Serve with tacos or nacho chips.
Notes
If desired, spread in a baking dish and cover with grated cheddar and monterey jack cheeses. Bake, uncovered for 30 minutes at 325.