Preheat your oven to 350°F (175°C).
Cut the Hawaiian rolls in half horizontally and place the bottom halves in a baking dish.
Layer the roast beef and cheese on the bottom halves of the rolls.
Place the top halves of the rolls back on.
In a small bowl, mix together the melted butter, Dijon mustard, Worcestershire sauce, poppy seeds, and onion powder.
Brush the butter mixture over the tops of the sandwiches.
Cover the baking dish with aluminum foil and bake for 15 minutes.
Remove the foil and bake for an additional 5 minutes, or until the tops are golden brown and crispy.
Remove from the oven and let cool slightly before serving.