Preheat oven to 425F. Remove giblets from inside of the chicken if necessary. Wash the chicken well inside and out and dry well.
Salt and pepper the cavity and place in the 1/2 sliced onion and the celery leaves. Fold the wings under the chicken. Julia says to fold them “akimbo”, which is I am sure what she meant.
Tie drumstick ends together. Rub the outside of the chicken with the butter and sprinkle on the salt and Herbs de Provence. Oil a cast iron pan or small roasting pan with a V-shaped rack with olive oil. Place chicken in the preheated oven and baste every 10 min. for 30 min.
Scatter the chopped onions and carrots around the chicken and reduce the oven to 350F. Continue to baste every 10 min. now including the vegetables.
The chicken will take an hour and 10 to 20 min. to reach an internal temperature of 170F. Remove chicken from the pan and let it rest on a cutting board while you make the sauce.
Remove vegetables from the pan and all the drippings but 1 Tbsp. Heat the pan on the stove and add the chicken and beef stock. Bring up to a rapid boil and reduce slightly. Add the fresh thyme leaves. Stir the cornstarch and port wine together and add to the sauce stirring constantly. Adjust seasoning with salt and pepper. If too thick adjust with more stock.
Slice chicken and serve over mashed potatoes with cooked green beans (or veggie of choice) and the roasted carrots and onions. Pour the sauce over and serve immediately. Gorgeous!!