Adapted from here.
Add garlic, shallots, celery, carrots and mushrooms to a medium bowl and toss with remaining olive oil. Take the skillet from the oven, then scrape all the vegetables on top. Add the peppercorns and carefully toss everything together. Return the skillet to the oven and roast until everything is deeply golden brown (35 to 40 minutes). Halfway through cooking, I like to flip the wings again and move around any veggies that aren’t getting golden.
Transfer the skillet from the oven to the stovetop, then transfer the wings to a large pot (you can leave the veggies in the skillet). Place the skillet over medium-high heat, add sherry and scrape up any brown bits stuck to the bottom of the skillet. Continue to cook until the sherry is almost completely reduced (about 2 mins).
Scrape everything from the skillet into the pot with the turkey, then add stock, parsley, thyme and bay. Bring to a gentle simmer over medium-high heat, reduce heat to medium-low and continue to simmer until the stock is reduced by 1/3 (about 35 to 40 minutes). While the stock is simmering, you’ll want to turn the wings occasionally as they poke up above the surface. Also, skim off any excess fat that pools on the surface.
Using a slotted spoon, remove big solids from the stock and discard. Pour the stock through a fine-mesh strainer into a large bowl or measuring cup. Discard any solids. Rinse out the pot and return the turkey stock back in. Bring to a simmer over medium heat, then whisk in the flour-butter mixture until smooth. Continue to simmer until the gravy is thick enough to coat the back of a spoon (about 6 to 8 minutes).
* Vinegar and MSG can break down flavor compounds in the gravy if added too far in advance. If you plan on making this gravy on Thanksgiving, then go ahead and add the vinegar and MSG right away. If you are making this gravy in advance (up to 5 days), then add the vinegar and MSG when you are reheating the gravy for Thanksgiving dinner.