Heat grill to medium-high or set oven to 400 degrees.
Mix the softened butter with 1 tablespoon of the au jus mix and salt to taste (you can also add garlic salt if you'd like!).
Make an odd number of slices in your loaf of bread, a little less than 1/2 thickness. Spread butter in-between each slice of bread Save any butter you have leftover.
Stuff every other slice of bread with a couple slices of roast beef and a slice of cheese. If desired, spread remaining butter mix over the top of your bread and anywhere else you think needs a little extra.
Wrap your sandwich loaf completely with foil and place on the grill for 5-10 minutes making sure to flip frequently so your bread doesn't burn. If using the oven, bake for 10 minutes. Either way, you just want to grill it long enough to warm up and melt cheese.
Meanwhile, make the au jus according to package directions (I added a little less water to make up for the 1 tablespoon added to the butter but do you!).
Remove sandwich from grill, open carefully, pull apart and serve with prepared au jus!