Mix the ground round, salt and pepper and mustard together lightly and form into 3 1-inch patties. Melt butter in skillet. Saute hamburgers on medium heat about 5 minutes each side. Remove and keep warm. Add lemon juice and Worcestershire sauce to pan and stir over low heat until well blended. Return patties to pan and let simmer in juices a few minutes while spooning juice over the top occasionally.