Preheat oven to 350 degrees F. Place bread cubes in bowl. Melt butter in a skillet set over medium heat. Add onions and celery and cook until tender. Mix the onions, celery and remaining stuffing ingredients with the bread cubes. Cool stuffing to room temperature.
Rinse chicken in cold water and pat dry. Season cavity with salt and pepper and loosely fill with stuffing. Place rest of stuffing in a small, lightly buttered casserole dish. Tie legs of chicken together with string and then place in a roasting pan. Sprinkle with salt, pepper and sage. Roast 90 minutes, or until internal temperature of the chicken reaches 170 degrees F when a meat thermometer is inserted the deepest part of the thigh. Remove chicken from pan, loosely cover with foil, and let rest 10 before carving.
Remove fat from pan-drippings and then place pan on the stove. Add 1 1/2 cups of chicken stock and bring to boil, scraping the bottom to dislodge the flavourful brown bits. Slowly whisk in flour/stock mixture. Simmer until thickened and then season to taste.