Preheat your oven to 425°F. In a large roasting pan, arrange the quartered onion, chopped carrots, celery, and smashed garlic cloves. Drizzle with approximately 2 tablespoons of olive oil and season with salt and freshly cracked black pepper. Toss to coat and spread evenly across the bottom of the pan.
Arrange the turkey neck, wings, or drumsticks directly on top of the vegetables. Drizzle with the remaining olive oil and season generously with salt and pepper. Scatter the fresh thyme, rosemary, and sage over the turkey and vegetables.
Transfer the roasting pan to the oven and roast uncovered for approximately 35–45 minutes, or until the turkey and vegetables are nicely browned and beginning to caramelize. This roasting process develops the rich, savory flavor that makes the gravy so delicious.
Carefully remove the roasting pan from the oven and pour in the dry white wine and chicken stock. Use a wooden spoon to scrape up any browned bits from the bottom of the pan, as these add incredible flavor to the gravy.
Return the pan to the oven and roast for an additional 35–45 minutes, allowing the liquid to simmer, reduce slightly, and absorb the flavors of the roasted turkey, vegetables, and fresh herbs. Add a splash of additional stock if the liquid reduces too quickly.
Remove the roasting pan from the oven. Pour the contents through a fine-mesh strainer into a large bowl or measuring pitcher, pressing gently on the vegetables to extract their flavorful juices. Discard the solids. Skim off excess fat from the surface of the strained liquid. You should have approximately 4 cups of flavorful turkey stock. If needed, add additional chicken stock to reach 4 cups.
In a large saucepan over medium heat, melt the butter. Sprinkle in the flour and whisk continuously for 2–3 minutes, until the mixture is smooth, lightly golden, and the raw flour smell has cooked off.
Slowly pour the warm, strained turkey stock into the roux, whisking constantly to prevent lumps. Continue adding the liquid gradually until fully incorporated.
Bring the gravy to a gentle simmer and cook for approximately 5–10 minutes, whisking occasionally, until thickened enough to coat the back of a spoon. Season with additional salt and freshly cracked black pepper to taste.
Serve immediately or allow the gravy to cool before transferring to an airtight container for refrigeration or freezing.