Coat slow cooker bowl with nonstick cooking spray. Rub roast with garlic salt and pepper. Place onions in bottom of prepared slow cooker. Add beef broth and Worcestershire sauce; place roast on top of onions. Cover and cook on HIGH for 6 hours or on LOW for 8 hours.
Heat broiler. Toast cut side of rolls for 2 to 3 minutes.
Carefully remove roast to a cutting board. Strain liquid from slow cooker into a bowl, reserving onions. Cut roast horizontally in half, then crosswise into slices. In a small bowl, blend mayonnaise and horseradish. Spread 1 1/2 tbsp mayo blend on each roll. Top with beef and some of the reserved onions. Serve each sandwich with 1/3 cup of the liquid (jus) for dipping.
