Ham and Swiss Sliders

Without separating rolls, cut rolls in half horizontally; arrange bottom halves of rolls in a greased 9x9-in. baking pan. Spread cut side of roll bottoms with horseradish sauce. Layer with ham and cheese; replace tops., In a small skillet, heat butter over medium-high heat. Add onion; cook and stir until tender, 1-2 minutes. Stir in remaining ingredients. Pour over rolls. Refrigerate, covered, several hours or overnight., Preheat oven to 350°. Bake, covered, 25 minutes. Bake, uncovered, until golden brown, 5-10 minutes longer.