Slice rolls in half horizontally. Spread mustard evenly on the bottom of the rolls.
Place rolls in a 9 x 13 baking dish.
Fold pastrami into thirds and place on each roll. Fold swiss cheese slices into thirds and place on top of pastrami.
Put top of roll on top.
In a small microwave safe bowl, heat butter and onion for 1 minute in microwave. Stir in garlic powder. Brush garlic butter mixture all over tops of rolls and on the sides until completely covered.
Cover with foil.
Bake for 20 minutes.
Remove foil and continue to bake for 6-8 minutes longer until tops are golden brown.