Step 1: Slice each onion into 1/2-inch-thick rings. Pop the centers out of the largest rings, leaving the outermost wall 2 layers thick. (Reserve the remaining onion for another use.)
Step 2: Brush the outside of the rings with maple syrup and generously season with the rub. Spiral bacon slices around each onion ring, overlapping the bacon slightly. (You’ll need about 2 pieces per ring.) Secure with a toothpick.
Step 3: Set up the grill for indirect grilling and preheat to medium. Grill the rings until the bacon is crisp and nicely browned, about 40 minutes.