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In a bowl, whisk the egg and milk. Add the bread crumbs, 1 tablespoon gravy mix and onion. Crumble the beef over the mixture and mix well.
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Shape into two patties, about 3/4 inches thick.
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Broil 3-4 inches from the heat for 6-7 minutes on each side or until the meat is no longer pink and a meat thermometer reads 160°.
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Place the remaining gravy mix in a small saucepan; stir in the water and mustard. Bring to a boil; cook and stir until thickened. Serve over the patties.