Combine egg and milk in a large mixing bowl, beating well. Add meat, onion, Worcestershire sauce, salt, and pepper; mix well. Shape into a 12- x 10- inch rectangle on a sheet of waxed paper.
Spoon stuffing over beef, leaving a 1/2-inch margin around edges. Beginning at short end, roll up meat jellyroll fashion, lifting waxed paper to help in rolling. Press seams and ends together to seal.
Place roll, seam side down, in a 13- x 9- x 2-inch baking pan. Combine catsup and water; pour over roll. Bake at 350° for 1 hour and 15 minutes. Remove to a serving platter. Garnish with toasted walnut halves and orange slices.