Season 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 and place in the cavity of the chicken, along with the thyme sprigs. Tie legs together with clean cotton kitchen strong. Place chicken, breast side up, on a rack in a roasting pan.
Cut remaining onion 1-1/2 onions into wedges. Trim 1/4 inch off each of the top of the garlic bulbs. In a large bowl combine onion wedges, garlic, carrots, celery, broth, olive oil, sage, bay leaves, thyme leaves, and remaining 1 teaspoon salt. Arrange vegetables around chicken; spoon liquid over chicken.
Roast in a 425 degrees F oven for 1-1/4 to 1-3/4 hours until chicken is no longer pink (180 degrees F in thigh), stirring vegetables a few times and adding small amounts of chicken broth if pan begins to get too brown.
Remove from oven and let rest 10 minutes. Remove bay leaves and sage sprigs. Serve with vegetables and pan juices.