
Grease a 9-inch square baking pan. Remove the entire loaf of rolls from the bag, and cut them in half horizontally without separating the rolls. Spread the horseradish over the roll bottoms. Layer with ham and cheese, and replace the roll tops.
In a small skillet or saucepan, melt the butter over medium-high heat. Add the onion and cook until tender, one to two minutes. Stir in the brown sugar, spicy brown mustard, poppy seeds, Worcestershire sauce and garlic powder.
Pour the butter topping over the prepared rolls. Cover the baking pan with aluminum foil, and refrigerate the rolls for several hours or overnight.
Remove the rolls from the refrigerator 30 minutes before baking, and preheat the oven to 350°F. Bake the rolls, covered, for 25 minutes. Uncover the rolls and bake an additional 5 to 10 minutes, until golden brown. Serve warm.