Combine onion, carrots, celery and vegetable oil in a large roasting pan and toss to coat. Place Turkey wings on top of vegetables

Roast in the preheated oven until turkey wings are browned and vegetables are caramelized and softened, 45 to 60 minutes or more

Transfer turkey wings and vegetables to a large stockpot. Place the roasting pan over stovetop burners set to medium heat Pour 3 Tablespoons water into the pan and bring to a boil, scraping up any browned bits. Transfer mixture to the stockpot and add 12 cups cold water, thyme and garlic

Bring turkey mixture to a boil.  Reduce heat to low and simmer, uncovered, until meat falls off the bone, about 3 hours.  Skim off turkey fat throughout the process and set aside 3 Tablespoons. I prefer to refrigerate the strained broth overnight. The fat rises to the top.  Remove and reserve 3 Tablespoons of the fat to make gravy

Strain turkey stock and reserve 4 cups of stock; discard all the solids. Freeze additional stock for future use.

Heat butter and 3 tablespoons turkey fat in a large saucepan over medium heat. Sprinkle in flour and cook, whisking constantly, until it begins to smell like cooked pie crust, about 2-3 minutes. If stock is cold then warm it in a small sauce pot. Slowly pour in 4 cups reserved turkey stock, whisking constantly.

Increase heat to medium-high and simmer gravy until thick and warmed through, about 5 minutes. Season with cayenne, salt and pepper.  Refrigerate for up to 4 days OR freeze until ready to use.