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Preheat the oven to 350 degrees F (175 degrees C). Grease a 9×13-inch baking dish.
Spread horseradish on the bottoms of the Hawaiian bread rolls. Top each with 1 slice roast beef and 1 slice Swiss cheese, then cover with top bread rolls. Place sandwiches in the prepared baking dish.
Melt butter in a saucepan over medium heat. Cook and stir onion, brown sugar, spicy mustard, Worcestershire sauce, and garlic powder in butter until onion is soft and translucent, about 5 minutes. Pour over sandwiches and sprinkle rolls with poppy seeds. Cover the dish with aluminum foil.
Bake in the preheated oven until heated through, about 25 minutes. Remove foil and continue baking until tops are lightly browned, about 10 minutes.
Recipe Tip
If preparing the sandwiches in advance, store the covered baking dish with sandwiches in the refrigerator for 3 to 12 hours until you’re ready to bake them.
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