Tear bread into small pieces and pour 1⁄2 cup of the butter over bread. Toss to coat. Let stand about 20 minutes, or until butter is absorbed. Add egg yolks, lobster, onion, Worcestershire sauce and mustard; mix well. Beat egg whites until stiff and fold into lobster mixture. Shape into 10 patties. In a large skillet, melt remaining 1⁄4 cup butter. Add patties and cook gently until golden brown on both sides.