In a large nonstick skillet over medium heat, melt the butter. Add the onions and saute, stirring occasionally, for about 12-15 minutes until the onions are soft and browned, starting to caramelize, but not at all burnt. Add the thyme, salt, and pepper; stir to combine. Take off the heat and set aside.

Preheat the oven to 350°F. Spray a 11×7-inch baking dish with nonstick cooking spray.

Do not pull the rolls apart. Instead, using a large serrated knife, slice the rolls in half (like you would a bagel) so you have a top and bottom "slab."

Place the bottom slab, cut side up, into the prepared baking dish. Top with half of the cheese. Add the roast beef. Top with the cooked onions, spreading them out. Add the remaining cheese, then add the top slab of 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, if preferred.)

Cover loosely with foil, making 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 3 cups of water.

Serve the rolls with an optional garnish of parsley and au jus on the side for dipping.