Heat oil or butter in a wide braising pan or casserole. Add onion and garlic and cook over medium heat for 3 to 5 minutes, stirring and being careful not to get too much color on the onions. Add the greens (be sure to wash carefully as they have a tendency to be gritty) and stir. Cover for about 5 minutes to wilt, then cook over medium low heat for at least 15 to 20 minutes, or until greens are tender and not bitter. Sprinkle with the vinegar, season with salt and pepper, stir, and remove from heat. Serve warm.