Heat grill to medium-high. In a large bowl, combine ground beef, onion, 3 tbsp of the barbecue sauce, the Parmesan, mustard and garlic powder. Gently mix until combined, then shape into 4 patties about the size of the sourdough slices.Advertisement
Brush sourdough on both sides with olive oil. Grill for 2 minutes per side, turning once. Transfer to a platter and top with Boston lettuce leaves.
Grill burger patties on oiled grill for 4 minutes. Spritz burgers with nonstick cooking spray and flip over. Grill an additional 4 minutes, or to desired doneness. Transfer to lettuce-topped bread and spoon 1 tbsp barbecue sauce onto each burger. Top each with 1 tbsp fried onions and serve with a mixed green salad, if desired.
Before grilling, indent the center of each patty with your fingertip to prevent shrinking and help hold shape while cooking.