Bring chicken bones, broth, carrot, onion, celery, garlic, peppercorns, bay leaf, and cloves to a boil in a 3- to 4-quart heavy saucepan, then simmer, partially covered, skimming froth, 30 minutes.
Pour stock through a large sieve into a bowl, discarding solids, and return to saucepan. Measure stock: If more than 2 cups, boil until reduced. Keep warm, covered.
Put oven rack in middle position and preheat oven to 350°F. Butter a shallow 3-quart glass or ceramic baking dish.
Heat 3 tablespoons butter in a large heavy skillet over moderately high heat until foam subsides, then sauté mushrooms with 1/4 teaspoon salt and 1/8 teaspoon pepper, stirring, until liquid mushrooms give off is evaporated and mushrooms begin to turn golden, about 8 minutes.
Melt remaining 3 tablespoons butter in a 2- to 3-quart heavy saucepan over low heat, then add flour and cook roux, whisking, 3 minutes. Add warm stock in a fast stream, whisking constantly, and bring to a boil. Reduce heat and simmer, whisking occasionally, 5 minutes. Add cream, Sherry, remaining 1/4 teaspoon salt, and remaining 1/8 teaspoon pepper and simmer over low heat, whisking occasionally, 10 minutes. Stir in truffle butter (if using).
Cook spaghetti in a large pot of >boiling salted water until al dente, then drain well.
Toss together spaghetti, mushrooms, and half of sauce in a large bowl, then transfer to baking dish. Stir together chicken meat and remaining sauce in same large bowl. Make a depression in spaghetti, then spoon chicken into it and sprinkle dish with cheese.
Bake until sauce is bubbling and top is lightly browned, about 30 minutes. Serve immediately.
*Available at some specialty foods shops and D'Artagnan (800-327-8246).
