Saute leeks, onion, and thyme in 1/2 cup butter in a large skillet until tender. Transfer leek mixture to a bowl; add hazelnuts and next 6 ingredients. Cover and chill.
Remove giblets and neck from turkey. Rinse turkey under cold water; pat dry. Pack about 7 cups leek mixture into cavity of turkey. Tuck legs under flap of skin around tail, or close cavity with skewers, and truss. Tie ends of legs to tail with cord. Lift wing tips up and over back; tuck under turkey.
Brush turkey with 2 tablespoons butter, and place, breast side up, on a rack in a roasting pan. Insert meat thermometer in meaty part of thigh. Cover with heavy-duty aluminum foil, and bake at 325° for 2 hours. Uncover and baste with remaining 2 tablespoons butter. Bake 2 hours or until meat thermometer registers 180°.
Spoon remaining leek mixture into a 1-quart casserole coated with cooking spray. Bake at 325° for 25 minutes. Serve with turkey. If desired, garnish with fresh fruit and thyme.