Place chicken in roasting pan over 3 chopped onions 3 celery ribs and 3 peeled chopped carrots. Loosen skin on chicken. Rub with herb mix under skin and all around. Stuff with 3 bay leaves, 6 cloves garlic, 1 bunch thyme and 2 lemons. Roast at 450 degrees for 15 minutes. Reduce heat. Cook 45 more minutes until 165 degrees internal temp is reached. Cover. Let rest 10 minutes.
To make gravy: Deglaze roasting pan with 5 cups chicken stock. Bring to boil. Knead 3 tablespoons flour with 3 tablespoons butter. Whisk into simmering liquid. Cook 5 minutes. Strain. Serve with chicken.