In a medium skillet over medium heat, melt 2 tablespoons of butter and cook the onions until lightly browned and soft. Set aside.
Preheat the oven to 350°F and grease a 9 x 13 baking dish with nonstick cooking spray.
Keeping the Hawaiian sweet rolls attached, run a serrated knife through the middle of the loaf, dividing the top and bottom halves.
Place the bottom half in the prepared baking dish and evenly lay on the cheese.
Scatter on the roast beef, followed by the onions.
Top with the top half of the rolls.
In a small bowl, melt the remaining 8 tablespoons of butter in the microwave (about 15 seconds). Stir in the Au Jus gravy mix, Worcestershire sauce, sesame seeds, garlic powder, and onion powder.
Pour sauce over the rolls and bake until the cheese has melted, about 15-20 minutes.