Heat the oven to 400°F. Roast the turkey wings or neck with the onion, carrot, celery, and garlic for 60 to 75 minutes, until the turkey turns deep brown.
Move everything to a stockpot. Add the chicken stock, water, thyme, rosemary, sage, parsley, and bay leaf.
Simmer for 45 minutes, then strain the liquid. Cool it slightly and separate the fat.
Melt 1/2 cup turkey fat or butter in a saucepan over medium heat. Whisk in the flour and cook the roux for 3 to 5 minutes, until it turns light golden and smells nutty.
Slowly whisk in the warm gravy base. Simmer until thick, then season with kosher salt and black pepper.
Cool the gravy, then refrigerate it for up to 3 days or freeze it.
Before dinner, reheat the gravy gently. Pour the turkey drippings and juices through the fat separator, then whisk the flavorful pan drippings into the hot gravy.
Blend briefly with an immersion blender only if you want a smoother texture.