In a large skillet over medium heat, melt 1 tablespoon of unsalted butter. Add the thinly sliced onions and sauté for 12-15 minutes, stirring occasionally, until they become soft and golden brown. Stir in minced thyme, kosher salt, and black pepper.

Preheat your oven to 350°F (175°C). Position a baking dish or a rimmed baking sheet near your prep area.

Slice the sweet rolls in half horizontally, keeping the tops attached to the bottoms. Place the bottom halves in the prepared baking dish.

Layer half of the provolone slices over the bottom halves, then add sliced deli roast beef, sautéed onions, and remaining provolone slices.

In a small bowl, combine 1/2 cup melted unsalted butter and 1 1/2 tablespoons dried onion soup mix. Brush or pour this evenly over the assembled sliders.

Cover the baking dish loosely with aluminum foil and bake for 25 minutes. Remove foil and bake for an additional 5 minutes or until tops are golden brown.

Prepare the au jus by adding 3 cups of water or beef broth to a saucepan and whisking until thickened, about 5-7 minutes.

Let the sliders cool for a few minutes, garnish with fresh parsley if desired, then slice and serve warm with au jus for dipping.