Sauté the onions in the butter. Mix bread cubes with onions, parsley, celery, 1 teaspoon salt, 1/2 teaspoon pepper, the thyme, stock and grapes.
Sprinkle the inside of the duck with salt and pepper. Fill crop and body cavities with grape stuffing. Close openings with skewers. Lace tightly with a strong string. Rub the outside of the skin with the lemon.
Place the duck on a rack in a shallow pan, such as a jelly roll pan. Bake for 2 to 2-1/2 hours.