1. In a small bowl, combine the horseradish, Worcestershire sauce and mayonnaise. Season to taste with salt and pepper.
(This can be made in advance and stored in the refrigerator for up to 3 days.)
2. Preheat the grill to medium-high.
3. Shape the ground sirloin into 4 patties, about 1/2-inch thick. Season the patties with salt and pepper.
4. Grill the patties on both sides until the burgers are just cooked through, about 5 minutes per side.
5. Toast the onion rolls on the grill.
6. Serve the burgers with sliced red onion and horseradish sauce on the toasted onion rolls.Serving Size: 1 burger