Preheat a grill to medium-high heat (400 degrees) or the oven to 400 degrees F.
Start by making the butter mixture. Combine the softened (not melted) butter, 1 tablespoon of the au jus gravy mix, Worcestershire sauce, dried minced garlic, and onion powder in a bowl. Stir until ingredients are completely combined and set aside.
Cut the baguette into equal parts of about 6-10 inches in length. Next, make an ODD number of thin slices (approx. 1/2 inch wide) into each piece of the baguette, cutting only about 3/4 the way through. It's important there are an ODD number of slices so that each sandwich has 2 pieces of bread.
Separate the butter mixture created in step 2 into 2 equal parts. Using a butter knife, spread a small amount of the mixture inside each slice of bread. Save half of the mixture for later.
Slice the individual pieces of cheese in half and place 1 piece, along with 1 piece of roast beef, in-between every two "slices" of bread. (Again, these will be individual pull-apart sandwiches, so you don't want cheese and meat in-between each slice).
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 each baguette section completely in foil. Make sure none of the bread is exposed.
Place in the preheated oven for 10 minutes or the preheated grill for 6-9 minutes (turning frequently) or until bread is toasty and cheese is melted. If you have a thicker baguette it will take longer.
While the sandwiches are cooking, prepare the au jus. Take the remaining au jus mixture and place in 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 slightly thicken. (You can do this on the side burner of a grill or even over a campfire OR make ahead of you bring these sandwiches camping)
Remove foil packs and open carefully to let the steam escape. Pull apart the sandwiches and enjoy with the au jus sauce.