Fortifying store-bought broth with turkey necks results in lip-sticking stock while the tannins in the wine combine with the proteins in the stock to remove the acidity and leave you with earthy, jammy fruit flavors.
Preheat oven to 425°F. Toss together turkey necks, onion, celery, carrots, and oil in a large bowl. Spread in a single layer on 2 rimmed baking sheets. Roast in preheated oven until browned, 1 hour to 1 hour and 15 minutes, turning once halfway through roasting.
Place roasted turkey necks and vegetables in a stockpot. Divide wine evenly between hot baking sheets, stirring and scraping up browned bits with a wooden spoon; add wine mixture to stockpot. Add drumstick, garlic, bay leaves, peppercorns, thyme, parsley, and broth. Bring to a boil over high. Reduce heat to medium-low, and simmer, skimming and discarding foam from top as needed, until reduced to about 6 cups, 1 hour and 45 minutes to 2 hours. Pour mixture through a fine wire-mesh strainer into a large heatproof bowl; discard solids. Let stand 10 minutes; skim off and discard fat.
Melt butter in a large saucepan over medium. Add flour; cook, whisking often, until golden brown, about 5 minutes. Gradually pour in hot stock mixture, whisking constantly, until smooth. Bring to a simmer over medium. Simmer, whisking often, until mixture thickens, about 15 minutes. Stir in salt and pepper. Hold rosemary sprig at 1 end, and swirl in gravy until it imparts a subtle herb flavor.
Gravy can be made through step 2 and refrigerated up to 2 days in advance.
Look for smoked turkey drumsticks refrigerated at the grocery store near the bacon or at your butcher.