Prepare gravy according to package directions.
Split hamburger buns, and butter the insides. Grill in a hot skillet until browned; set aside.
In the same skillet, melt 1 tbsp (15 mL) of butter; sauté the steak and onions for 5 minutes.
Pour in 3 oz (85 g) of gravy, and simmer until liquid is reduced by half.
Dress the bottom halves of the buns with relish and mustard.
Top with steak and onions, and a slice of cheese. Serve with plenty of napkins!