Preheat the oven to 350˚F. Lightly spray a 9×13 inch baking dish with cooking spray oil.

In a large skillet over medium heat, melt butter with oil for the onions. Once melted, add onions and season lightly with salt. Sauté onions, stirring occasionally, until brown and caramelized. This will take about 15 to 30 minutes. Set aside.

Without separating the rolls, slice them in half using a long knife. Place the bottom half of the rolls in the prepared baking dish.

Top with the roast beef slices in an even layer. Add cheese slices on top and spread out the caramelized onions on top of the cheese. Cover with the other half of the rolls.

In a small bowl, whisk together melted butter, sugar, mustard, Worcestershire sauce, garlic powder, onion powder, and dried thyme. Pour the butter mixture over the tops of the Hawaiian rolls. Use a brush to make sure that all sides of the rolls are covered with the butter.

Cover the baking dish with aluminum foil. Bake for 25 minutes.

Remove foil. Bake for an additional 7 to 10 minutes until the tops are lightly browned.