1. Heat the olive oil in a heavy 10- to 12-inch skillet, add the onions and cook them over moderate heat, stirring constantly, for 2 or 3 minutes.
2. When they are transparent but not brown, stir in the cabbage, tomatoes, vinegar, salt and a few grindings of pepper.
3. Simmer uncovered, stirring frequently, for 20 minutes, or until the cabbage is tender. Then stir the sugar into the cabbage and cook a minute or 2 longer.
4. Serve in a heated bowl, either as a vegetable accompanying a fish or meat course or as a separate dish preceding the main course.
