In a large bowl, combine turkey, barbecue sauce, onion, bread crumbs, garlic, seasoning salt and pepper just until combined. Form into 4 large patties (they will shrink as they cook so you want to make them slightly larger than your buns), and place on a plate or baking sheet in the refrigerator for 2-3 hours or in the freezer for 30-60 minutes — this will help them not to firm up slightly.

Preheat the grill to medium-high, or about 400 degrees F. Grill burgers on each side for 5-6 minutes until an internal temperature of 165 degrees F is reached.

Brush with additional ketchup sauce and serve as desired.