Coat a nonstick skillet with cooking spray; add 1 tablespoon margarine. Place over medium heat until margarine melts. Add leeks, onion, thyme, and 1/2 cup broth. Bring to a boil. Cover, reduce heat, and simmer until vegetables are tender. Uncover and cook over high heat until liquid evaporates. Transfer leek mixture to a bowl; add croutons and next 3 ingredients. Add 1 1/4 cups broth; 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 wingtips up and over back; tuck under turkey.
Coat turkey with cooking spray; place, breast side up, on a rack in a roasting pan coated with cooking spray. Insert meat thermometer in meaty part of thigh. Cover with heavy-duty aluminum foil; bake at 325° for 2 hours. Uncover and baste with melted margarine. 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 skinned turkey slices.