Step 1

Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.

Step 2

Spread horseradish on the bottoms of the Hawaiian bread rolls and cover each with 1 slice roast beef and 1 slice Swiss cheese. Cover with top bread rolls.

Step 3

Melt butter in a saucepan over medium heat. Cook and stir onion, brown sugar, spicy mustard, Worcestershire sauce, and garlic powder until onion is soft and translucent, about 5 minutes. Pour over Hawaiian rolls and sprinkle rolls with poppy seeds. Cover with aluminum foil.

Step 4

Bake in the preheated oven until heated through, about 25 minutes. Uncover and bake until tops are lightly browned, about 10 minutes.