For onion topping, cut onions into 12-inch-thick slices. On the rack of an uncovered grill, grill onions directly over medium coals for 15 to 20 minutes or until tender, turning once. Grill burger patties directly over the coals alongside the onions for 4 to 5 minutes per side or until heated through. Brush grilled onions with the salad dressing.
Meanwhile, for spinach topping, in a large skillet cook and stir the spinach and garlic in hot olive oil over medium-high heat for 30 seconds or until spinach is just wilted. Remove from heat. Stir in feta cheese.
To serve, place onion slices on bottom of buns. Top with grilled burger patties, spinach mixture, and bun tops. Makes 4 servings.