Sirloin Steak with Golden Onions

Steak and Onions: Coat a large nonstick skillet with nonstick spray; heat on medium-high. Sprinkle steak with salt and pepper. Add to skillet. Scatter onions around steak. Cook over medium-high heat, turning steak once and stirring onions often, 12 minutes, or until steak is medium-rare and onions are golden and tender. (For more well-done meat, cook steak longer and remove onions first.) Transfer steak to cutting board; let stand 5 minutes. Meanwhile stir Horseradish Sauce ingredients in a small bowl to combine. Slice steak thinly against the grain. Reserve and wrap 11/2 cups meat and 1 cup onions. Refrigerate up to 3 days. Serve remaining steak and onions with the Horseradish Sauce.