Combine first 4 ingredients and juice of 1 lemon half; set aside.
Remove giblets and neck from turkey; reserve for other uses. Rinse turkey with cold water; pat dry. Tie ends of legs to tail with cord if legs are not tucked under flap of skin. Lift wingtips up and over back, and tuck under bird.
Place turkey on rack of a roasting pan, breast side up; rub turkey with other lemon half, squeezing juice over turkey. Spray turkey with cooking spray. Insert meat thermometer into meaty part of thigh, making sure it does not touch bone.
Bake at 325 degrees for 3 hours or until meat thermometer reaches 185 degrees, basting every 30 minutes after the first hour with Madeira mixture. If turkey starts to brown too much, cover with aluminum foil.
When turkey is 2/3 done, cut the cord or band of skin holding drumsticks to tail; this will ensure that thighs are cooked internally. Turkey is done when drumsticks are easy to move up and down. Let stand 15 minutes before carving.
Measure remaining basting mixture and pan juices from turkey. Using 1 tablespoon cornstarch to each cup of drippings, combine cornstarch and 1/4 cup water, stirring until smooth; stir into pan drippings. Bring to a boil; boil 1 minute, stirring constantly. Serve.