Preheat oven to 425 degrees F. Rinse chicken body cavity; pat dry with paper towels. Sprinkle chicken inside and out with 1-1/2 teaspoons of the salt and the pepper. Cut one-half of one of the onions into two pieces; place onion pieces and thyme sprigs in body cavity of chicken. Skewer neck skin to back; tie legs to tail. Twist wing tips under back. Place chicken, breast side up, on a rack in a shallow roasting pan.
Cut the remaining 1-1/2 onions into wedges. Cut off the top 1/4 inch of garlic bulbs to expose ends of individual cloves. Leaving garlic bulbs whole, remove any loose, papery outer layers. In a large bowl combine onion wedges, garlic bulbs, carrots, celery, 1/4 cup broth, oil, bay leaves, sage sprigs, thyme leaves, and the remaining 1 teaspoon salt. Arrange vegetables around chicken; spoon liquid over chicken.
Roast, uncovered, for 1-1/4 to 1-3/4 hours or until drumsticks move easily in sockets and chicken is no longer pink (180 degrees F), stirring vegetables a few times and adding small amounts of additional broth if pan begins to get too brown. Remove from oven.
Cover and let stand for 10 minutes before carving chicken. Remove and discard bay leaves and sage sprigs. Serve chicken with vegetables and pan juices.