Melt the butter in a large skillet over medium high heat. Add in the sliced onions and sauté for 10-15 minutes until the onions are soft and slightly browned. Season with salt and pepper and remove from the heat.
Preheat the oven to 350 degrees Fahrenheit and spray a 9X13 baking dish with a non-stick cooking spray.
Slice the rolls in half lengthwise with a serrated knife to make sandwiches. Remove top and set aside.
Place the bottom pieces into the prepared baking dish. Top with half of the sliced cheese.
Layer the roast beef on top of the cheese and layer the onions on top. Top with the remaining cheese slices.
Place the tops of the rolls onto the sandwiches. Stir together the melted butter and dried onion soup mix. Brush this mixture on top of the rolls.
Cover with foil and bake for 25 minutes. Remove the foil and cook for 5 more minutes to brown the top of the rolls slightly.
While the rolls are baking, prepare the au jus gravy mix based on the package instructions with the water.
Serve the rolls topped with fresh parsley and the au jus sauce for dipping. Enjoy!
Notes
Refrigerate the leftovers in an airtight container for up to 5 days.