In a large skillet over medium heat, melt 3 tablespoons of unsalted butter. Add thinly sliced onions and sauté for 12-15 minutes, then stir in minced thyme, kosher salt, and black pepper. Remove from heat and set aside.

Preheat your oven to 350°F (175°C) while the onions caramelize.

Slice the sweet rolls horizontally in half, and place the bottom half in a large baking dish.

Layer half of the provolone cheese, roast beef, caramelized onions, and the remaining provolone cheese on the bottom half of the rolls.

In a small bowl, combine melted butter and dried onion soup mix. Brush the mixture over the assembled sliders.

Cover the baking dish with foil and bake for 25 minutes. Remove foil and bake for an additional 5 minutes.

While baking, prepare the au jus according to the package instructions by combining the mix with 3 cups of water.

Remove the sliders from the oven and garnish with parsley if desired. Serve warm alongside au jus for dipping.