Preheat oven to 400°F / 177°C.
Make stock: Pour oil in the bottom of a Dutch oven or large oven-safe pan with tall sides. Add turkey parts and then onion, celery, and carrots. Season generously with salt and pepper. Transfer to heated oven and roast, uncovered, until everything is starting to brown lightly, about 60 minutes.
After roasting, transfer pan to the stovetop (be careful - the handles will be very hot!).
Add water and bay leaves and bring to a boil over high heat.
Once the stock is boiling, reduce heat to a low simmer. Simmer, uncovered, for 60 minutes. (Be sure to keep the stock at a low simmer to prevent too much liquid from evaporating.)
Taste stock and season, to taste with some more salt.
Pour stock through a fine mesh strainer (or any strainer, lined with cheesecloth).
Make gravy: Heat butter over medium heat in a skillet or saute pan.
When butter is melted and starting to turn golden, whisk in flour until no dry spots remain. Cook butter and flour together until golden brown, about 3 minutes.
Slowly add stock while whisking.
Bring to a boil and boil until thickened to your preferred consistency, 6 to 10 minutes.
Taste and season with some more salt if needed.
Refrigerate and reheat: Cover gravy with plastic wrap, pressing it down against the surface of the gravy. Refrigerate for up to 3 days.
To reheat: When ready to serve, warm gravy in a saucepan over medium heat, whisking frequently. Add some more stock or water if needed to thin it out. Before serving, be sure to taste it once more and season if needed. (Note: If you have pan drippings from a roasted turkey, you can whisk in about 1 Tbsp of the drippings as the gravy reheats. This will add even more of the classic roasted turkey flavor.)