Heat 2 tablespoons of the oil in a skillet over medium-high heat and fry the slices, in batches if necessary, adding more oil as needed, for 2 to 3 minutes per side, until the inside is just slightly pink. Cut into a slice to make sure you are not overcooking it. Remove the slices from the pan as they are done and cover loosely to keep warm. Pour off all but 2 tablespoons of oil from the pan. Reduce the heat to medium and add the onions. Cover and cook until soft, about 5 minutes, stirring from time to time. Stir in the garlic and mushrooms and cook for 5 minutes, stirring occasionally. Add the parasley, sherry or wine, and mustard, bring to a boil, and cook until the sauce begins to turn syrupy. Taste for salt and pepper. Add the optional butter if you want a smoother, richer sauce.