Heat the oil, add the minced onion and salt. Cook until the onions are fragrant and just beginning to change color. This is not so much frying as it is simmering in hot oil.
Add the beans, make sure to carefully coat every bean in the hot oil, and cook for a couple of minutes
Add the chopped tomatoes and cook for 10-20 minutes, until the beans just lose their bright green and are fully cooked.