Preheat an oven to 350℉.
Add 2 tablespoons of the butter to a skillet over medium high heat. Once it has melted completely, add the sliced onions, salt and pepper. Sauté for 15 to 20 minutes, stirring frequently, until they are softened and have turned a golden brown, caramelized color. Remove from the heat and set aside.
6 tablespoons unsalted butter, divided, 1 large yellow onion, thinly sliced, 1 teaspoon kosher salt, ½ teaspoon black pepper
Leaving the rolls connected, use a serrated knife to slice them in half, leaving a large top and bottom piece.
1 12-count package Hawaiian sweet rolls
Melt the remaining butter and mix in the Dijon mustard. Using a pastry brush, coat the bottom of a 13 x 9 baking dish with half of the butter mixture. Place the bottom piece of the rolls into the baking pan. Layer evenly with the roast beef, then the onions, followed by the provolone cheese. Place the top half of the rolls over the cheese, then brush with the remaining melted butter mixture.
2 teaspoons Dijon mustard, ¾-1 pound deli roast beef, sliced, 6 slices provolone cheese
Cover the dish with aluminum foil, then bake for 15 minutes until the cheese is completely melted. Uncover and bake an additional 5 minutes until the tops of the rolls are lightly toasted.
While the rolls bake, add the ingredients for the jus to a saucepan over medium high heat. Cook for 8 to 10 minutes until it has reduced slightly. Adjust seasonings with more salt and pepper, if desired. Serve alongside the rolls while still warm.
1 ½ cups beef stock, 1 tablespoon Worcestershire sauce, 1 teaspoon onion powder, 1 teaspoon garlic powder, 2 tablespoons fresh thyme, chopped