1. Thaw turkey, if frozen. Remove neck and giblets. Rinse turkey body cavity.
2. For brine, in a large stockpot combine 4 cups water, salt, and sugar. Heat and stir just until salt and sugar are dissolved. Remove from heat. Stir in remaining 20 cups water and the ice. For food safety, cool brine to 45°F.
3. Place turkey in stockpot with brine. Add 2 oranges, 1 lemon, 1 onion, bay leaves, and peppercorns. Refrigerate 12 to 24 hours, rotating turkey occasionally if not entirely covered by brine.
4. Remove turkey from brine 1 hour before roasting. Rinse turkey inside and out with cold water; pat dry.
5. Preheat oven to 325°F. Spray a roasting rack with cooking spray. Place rack in a shallow roasting pan. For butter rub, combine 4 Tbsp. butter, oil, sage, thyme, orange zest, and garlic.
6. Place turkey, breast side up, on cutting board. Slide your fingers under the breast skin to loosen the skin from meat. Apply 2 Tbsp. butter rub underneath skin.
7. Place remaining orange, lemon, and onion pieces in turkey cavity. Skewer neck skin to back. Tie drumsticks together with kitchen string; twist wing tips under back. Place turkey, breast side up, on rack in roasting pan. Apply remaining butter rub to turkey. Loosely cover with foil.
8. Roast for 3¾ hours. Meanwhile, in a small pan, combine orange marmalade, orange juice, and agave nectar. Bring to a boil; reduce heat. Gently boil, uncovered, 15 to 20 minutes or until reduced to 1⅓ cups, stirring occasionally. Stir in remaining 2 Tbsp. butter. Remove ⅓ cup to brush on turkey; reserve remaining glaze for serving.
9. After the initial 3¾-hour roasting period, remove foil. Brush with 3 Tbsp. glaze. Roast for 15 to 45 minutes more or until internal temperature of thigh reaches 175°F. If necessary, cover with foil to prevent overbrowning.
10. Remove turkey from oven. Lightly brush with remaining 2 Tbsp. glaze. Cover with foil; let stand 20 minutes. Remove and discard citrus and onion from cavity before carving. Garnish as shown or as desired. Serve with reserved glaze.

Each serving contains: 430 calories, 22 g fat, 7 g saturated fat, 0 g trans fat, 179 mg cholesterol, 221 mg sodium, 10 g carbohydrates, 0 g fiber, 5 g sugar, 50 g protein. Daily values: 2% vitamin A, 5% vitamin C, 1% calcium, 17% iron.