In a large pot on high heat saute the onion in the olive oil until tender but not brown. Add the garlic. Continue to cook for 1 to 2 minutes. Add the tomatoes, mussels, and parsley. Cover and cook for 5 minutes or until all of the mussels open. Discard the unopened ones and with a large spoon stir the mussels around. Add the ouzo and taste sauce for seasoning.