In a large skillet over medium heat, melt the butter. Add the onions and coat in the butter. Cook, stirring occasionally, for about 12-15 minutes until the onions are soft and browned but not burnt.
Add the thyme, salt, and pepper, stir to combine, take off the heat, and set aside.
Preheat the oven to 350°F.
Take the rolls out of the package, leaving the rolls intact. Take a large knife and slice the rolls across so you have the tops and bottom separated. Place the bottom, cut side up into an 11x7 baking dish.
Top with half of the cheese.
Add the roast beef.
Top with the onions, spreading them out.
Add the remaining cheese, then add the tops of the rolls on top.
In a small bowl stir together the melted butter and onion soup mix. I like to brush this mixture on top of the rolls to make sure it’s evenly distributed but you can just pour the butter sauce on top.
Cover loosely with foil, you want to make sure the foil is not touching the tops of the rolls and bake for 25 minutes. Take the foil off and bake an additional 5 minutes to brown the rolls.
While the rolls are baking, prepare the au jus per the package directions with the water. Serve the rolls with an optional garnish of parsley and au jus on the side for dipping.
