Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
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.
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.
Bake in the preheated oven until heated through, about 25 minutes. Uncover and bake until tops are lightly browned, about 10 minutes.