Preheat oven to 350 degrees F.

Roast Turkey: Place turkey wings, onions, carrots and celery on a rimmed baking sheet. Drizzle with olive oil and rub all over the turkey and veggies. Season with kosher salt and ground black pepper. Pour 2 cups of stock in the pan. Place in the oven, middle rack, and let roast approx. 1 hour. 

Check Liquid Levels: Check periodically and add additional stock to the pan as it starts to cook down (don't let the pan dry out). 

Simmer: Remove pan from the oven. Transfer the turkey, veggies and any drippings to a 5.5qt (or bigger) dutch oven along with 4-5 cups of stock. Cover and let simmer approx. 60 minutes. Add additional splashes of stock if it starts to cook down. You should end up with approx. 3 cups of liquid. 

Strain Liquid: Remove the turkey and bigger pieces of veggies. Strain remaining liquid. 

Make Gravy: Bring a large, deep cast iron skillet to medium high heat. Add butter and let melt. Add flour and whisk to combine. Slowly add strained liquid 1/4 cup at a time and then whisk to combine. Continue until all the liquid has been added. Let simmer a few minutes as it thickens. 

Adjust: Taste and add additional salt, as needed. Add additional splashes of broth or water to thin slightly. Let simmer to thicken more. 

Let Cool + Freeze: Remove from heat and let cool completely. Transfer to a freezer-safe container (make sure to leave room - the liquid will expand as it freezes) and label clearly. 

To thaw, remove from freezer and let sit in the refrigerator overnight. Transfer to a medium sauce pan and warm slowly over medium heat, stirring fairly continuously.