Cut the onion into a small dice. Cut olives into small pieces. Cut tomatoes into large cubes. Chop greens. Heat olive oil in a saucepan, fry the onions until transparent. Add olives and tomato, simmer over low heat for 2 minutes. Salt pepper to taste, simmer over low heat for 5 minutes under a closed lid. Add greens to mix the sauce and turn off the heat. Serve the finished sauce to the table.