Preheat the oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish.

Heat 2 tablespoons butter and the olive oil in a skillet over medium heat. Add the sliced onion and cook for 8 to 10 minutes, stirring occasionally, until softened and lightly caramelized.

Stir in 1 teaspoon Worcestershire sauce, 1 teaspoon garlic powder, salt, and pepper. Cook for 1 minute more, then remove from the heat.

In a small saucepan, whisk together the beef broth and au jus gravy mix. Heat over medium heat until hot, then reduce the heat to low to keep warm.

Place the bottom halves of the slider buns in the prepared baking dish. Layer half of the roast beef over the buns, then add the caramelized onions and provolone cheese. Top with the remaining roast beef and the bun tops.

In a small bowl, mix the melted butter, Dijon mustard, 1 teaspoon Worcestershire sauce, onion powder, and 1/2 teaspoon garlic powder. Brush or spoon the mixture evenly over the tops of the buns. Sprinkle with sesame seeds if using.

Cover the dish with foil and bake for 15 minutes. Remove the foil and bake for 5 to 7 minutes more, until the cheese is melted and the tops are lightly golden.

Serve warm with the hot au jus for dipping.