In a large bowl, whisk egg and milk. Add the bread crumbs, 1 tablespoon gravy mix and onion. Crumble beef over mixture and mix well. Shape into two patties, about 3/4 in. thick. Place in a small baking pan.
Broil 3-4 in. from the heat for 6-7 minutes on each side or until a thermometer reads 160° and juices run clear.
Place the remaining gravy mix in a small saucepan; stir in water and mustard. Bring to a boil; cook and stir until thickened. Serve with patties.