Caramelize the onions. In a large skillet over medium heat, melt 3 tablespoons of butter. Add the sliced onions and cook, stirring occasionally, for 12–15 minutes until soft, golden brown, and caramelized.
Season the onions. Stir in the fresh thyme, kosher salt, and black pepper. Remove from the heat and set aside.
Prepare the rolls. Preheat the oven to 350°F (175°C). Without separating the rolls, slice the entire package horizontally into a top and bottom half. Place the bottom half, cut side up, into an 11×7-inch baking dish.
Assemble the sliders. Layer half of the provolone cheese over the bottom rolls. Evenly arrange the sliced roast beef on top, followed by the caramelized onions. Cover with the remaining provolone cheese, then place the top half of the rolls over the filling.
Make the butter topping. In a small bowl, combine the melted butter with the dried onion soup mix. Brush generously over the tops of the rolls or pour evenly across the surface.
Bake the sliders. Cover loosely with aluminum foil, making sure it doesn’t touch the tops of the rolls. Bake for 25 minutes. Remove the foil and bake for an additional 5 minutes, or until the tops are golden brown and slightly crisp.
Prepare the au jus. While the sliders bake, whisk together the au jus gravy mix and water according to the package directions. Simmer until hot.
Serve. Garnish the sliders with chopped parsley if desired. Serve warm with individual bowls of au jus for dipping.