Preheat oven to 425 degrees. Lightly oil 10 x 17-inch baking pan.
Wash and dry chicken, remove excess fat, trim wings. Massage chicken with salad dressing, making sure dressing is rubbed into all surfaces of the bird. Turn breast side up. Put half bunch of rosemary into cavity with quartered peeled onion. Scatter rosemary and garlic cloves in baking pan to make bed. Place chicken, legs loosely tied, on herb bed. Brush with any remaining balsamic vinaigrette.
Roast 45 minutes, brushing with pan juices every 15 minutes, until meat thermometer registers 160 degrees. Remove from oven. Rest on platter covered lightly with aluminum foil for 15 minutes.
Carve and serve with pan juices and soft garlic cloves over raw baby spinach leaves.