Rinse the chicken quickly under cold water. Dry thoroughly. Rub the cavity with the cut lemon, then sprinkle with salt and pepper to taste. Put the lemon, the onion, and bay leaf in the cavity and truss the chicken.

In a casserole large enough to hold the chicken, heat the oil and 2 tablespoons of the butter and brown the chicken over moderate heat on all sides, turning it carefully to avoid breaking the skin. This will take around 30 minutes. After 15 minutes, add the leek and rosemary and continue browning the chicken, stirring the leeks to make sure they don't burn.

Add the garlic and stir it for 1 minute. Add the wine, raise the heat to moderately high, and reduce the wine for 1 or 2 minutes. Add the tomatoes, stock, 1 teaspoon of salt, and pepper to taste. Bring to a simmer, cover, and bake in a preheated 325-degree oven until the chicken is tender, about 1-1/2 hours.

In a kettle, bring 5 quarts water to a boil and add 1 tbsp of salt and the vegetable oil. Cover and keep at a simmer so that it will be ready when the chicken is done.

When the chicken is tender remove it to a platter, rub it with the remaining tbsp butter, and sprinkle with parsley. Place it in a warm oven, with the heat off, until ready to serve as a main course after the pasta.

Add the fusilli to the boiling water, stir, and cook until at dente. Drain thoroughly. While the pasta is cooking, put the casserole over high heat and reduce the sauce until it has thickened. Set aside 1/2 cup of the sauce to serve with the chicken.

Toss the pasta in the sauce (reheated if necessary) in the casserole. Serve in heated bowls with a bowl of freshly grated Parmesan on the side.