Preheat a grill, oven, or fire pit to medium-high heat (400 degrees F).
Prepare the au jus butter: Combine the softened butter, 1 tablespoon of au jus gravy mix, Worcestershire sauce, dried minced garlic, and onion powder in a bowl. Stir until ingredients are completely combined and set aside.
Using a butter knife, spread a small amount of the au jus butter inside each slice of bread. Save half of the mixture for later.
Place 1 piece of cheese along with 1 piece of roast beef, in between every two slices of bread. (You should have 7-8 sandwiches side by side with no fillings in between.)
Take the other half of the butter mixture and brush it along the tops and sides of each of the sandwiches with a pastry brush.
Wrap the entire loaf completely in foil. Make sure none of the bread is exposed.
Place wrapped loaf in a preheated oven for 10 minutes or a preheated grill or fire pit for 6-9 minutes (turning frequently) or until bread is toasty and cheese is melted.
Meanwhile (or ahead of time), prepare the au jus. Take the remaining au jus mix (about 1 tbsp.) and add to a small saucepan over medium heat. Add 2 cups cold water and whisk. Bring to a boil and then reduce the heat to a simmer to allow it to thicken slightly. (You can also do this on the side burner of a grill or over a campfire.)
Remove foil-wrapped sandwich loaf and open carefully to let the steam escape. Pull apart the sandwiches and enjoy with the au jus dipping sauce.
Recipe adapted from https://www.chelseasmessyapron.com/foil-pack-french-dip-sandwiches