Turkey: In a saucepan, combine turkey neck and giblets with half each of onion, carrots, and celery; bay leaf; and 6 cups water. Season with salt and pepper; bring to a boil, then reduce heat to low and simmer 1 hour. Strain through a mesh sieve into a bowl (you should have at least 4 cups stock). If desired, pull meat from neck and chop giblets into bite-size pieces; reserve for gravy.

Meanwhile, preheat oven to 425 degrees. Combine 2 tablespoons salt and 3/4 teaspoon pepper. Season both cavities of turkey with half of salt-pepper mixture. Stuff large cavity with half of orange, half of garlic, and thyme sprigs. Tie legs together with kitchen twine. Stuff neck cavity with remaining orange half. Rub butter evenly all over skin, then season with remaining salt-pepper mixture. Place remaining onion, carrot, celery, and garlic in a roasting pan in a single layer. Top with a roasting rack. Transfer turkey to rack. Pour 1 cup stock into pan.

Roast turkey 1 hour, adding water or more stock to pan, 1/4 cup at a time, if bottom becomes dry and begins to blacken. Meanwhile, boil maple syrup in a saucepan until reduced by half, about 10 minutes. Remove from heat; stir in mustard and thyme leaves.

Reduce oven temperature to 350 degrees; continue roasting turkey until a thermometer inserted into thickest part of thigh (near but not touching bone) registers 150 degrees, 30 to 35 minutes. Brush with maple-syrup mixture (if it has begun to solidify, rewarm over low heat first) and continue roasting, basting every 15 minutes with more maple-syrup mixture, until thermometer registers 165 degrees, 35 to 45 minutes more, depending on bird size. (Reserve any extra syrup mixture for gravy.) Transfer turkey to a carving board or platter; let stand at least 20 minutes.

Gravy: Meanwhile, remove rack, onion, carrots, celery, and garlic from roasting pan; discard vegetables. Place pan across two burners; bring to a simmer over medium-high heat. Add wine; boil until mostly evaporated, about 1 minute. Add 2 1/2 cups stock to pan; return to a boil. Whisk remaining 1/2 cup stock into flour. Slowly add flour mixture to pan, whisking constantly; continue boiling until thickened slightly, 2 to 3 minutes. Whisk in any remaining maple-syrup mixture. Season with salt and pepper. Strain gravy through a mesh sieve into a bowl. Stir in reserved neck meat and giblets, if desired. Carve turkey; serve with gravy.