In a large bowl, combine the first 5 ingredients. Shape the mixture into 4 patties, each about 1" thick. Heat 1 tablespoon of the butter with the oil and mustard in a heavy skillet over moderate heat for about 1 minute. Add the patties and cook, uncovered, 3 minutes on each side for rare, 4 minutes for medium, and 6 for well done. Remove the patties to a warm platter. Add the lemon juice and Worcestershire sauce to the skillet, and stir for 30 seconds. Remove the skillet from the heat, and stir in the parsley and the remaining butter. Pour sauce over patties and serve.