Combine first 4 ingredients in a medium bowl; stir well. Shape mixture into 4 (1/2-inch-thick) patties.
Cut peppers in half lengthwise; remove and discard stems, seeds, and membranes.
Combine Italian dressing and garlic. Brush pepper halves and onion slices with dressing mixture.
Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place meat patties, pepper halves, and onion slices on rack; grill, covered, 5 minutes on each side or until done, basting occasionally with dressing mixture. Brush focaccia rounds lightly with dressing mixture. Place on grill rack; grill 2 minutes on each side or until lightly toasted.
To serve, place focaccia rounds on serving plates; arrange 3 spinach leaves on each. Top each with a meat patty, onion slice, pepper half, and 2 basil leaves.