In saucepan over medium heat, combine steak sauce and minced garlic. Stir until bubbly. Brush 4 tbsp onto both sides of sirloin steak. Keep remaining sauce.
Cut potatoes into quarters lengthwise. Cut off top and bottom of onions making a flat spot. Cut onions in half through their equators. Brush olive oil onto the potatoes and onion.
Place steak and vegetables on grill over medium heat. Direct grill with lid open until steak is cooked to desired degree of doneness and vegetables are tender. Turn steak and vegetables occasionally. Add sear marks. During the last 5 minutes of grilling, brush vegetables with remaining sauce. Remove steak from grill, trim fat, and carve steak crosswise into slices. Serve with vegetables and remaining warm sauce.