Butter the bottom of a 9 x 13 baking dish with the softened unsalted butter.
Without separating the rolls, slice the package of rolls in half (see video).
Place the bottom half of the rolls in the baking dish. Top with the 6 slices of the provolone cheese, roast beef, french fried onions, remaining 6 slices of cheese, and the other half of the rolls.
In a small bowl, melt the butter in the microwave. Mix in 1 tablespoon of the au jus seasoning mix, Worcestershire sauce, sesame seeds, dried minced garlic, onion powder, and seasoned salt **(I like things salty! If you don't leave this out). Whisk until well combined.
Top the rolls evenly with this mixture (you don't need to use all of it, we usually use about 3/4ths) but just make sure all the rolls are entirely coated. If you have leftover put it in the fridge to top steak or bread at another time.
Bake the sliders uncovered until the cheese is gooey and the tops of the rolls are golden brown -- about 15-20 minutes.
Remove and cut the sliders with a very sharp knife.
While the sliders are baking, 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.
Serve with the sandwiches.
*I like to get rare deli roast beef because as it cooks in these sliders it becomes medium-medium+ **I like things salty!! If you aren't a fan of too much salt, omit this addition.