Step 1Make a beurre manié: Knead together the unsalted butter and flour to form a sort of paste. Roll the paste into a log about 1/2-inch thick and wrap tightly in plastic wrap. Store in the freezer.
Step 2In a large saute pan heated over medium-high heat, add the oil, then the turkey bones and chicken legs, in batches (they will need to be cooked in a single layer), until browned, 10 to 15 minutes for each batch. Add additional oil if needed for each additional batch. Remove the bones and legs (save the leg meat for another use).
Step 3Reduce the heat to medium-low and add the butter to the pan, cooking until it is melted and lightly browned. Stir in the shallots, along with a little extra oil if desired so there is plenty of fat in the pan. Cook the shallots, stirring frequently, until they are golden-brown, 5 to 7 minutes. Add the white wine and cook, scraping any flavoring from the bottom of the pan, until the wine is evaporated or absorbed into the shallots.
Step 4Add about 1½ cups warm broth and simmer until the broth is reduced to a thick, jam-like consistency. Repeat this process twice more, reducing until the broth is thickened and jam-like in consistency.
Step 5Add the remaining stock and reduce by half. Add the bay leaves, sage, thyme and rosemary to the pan. Season with 1/4 teaspoon salt and several grinds of black pepper, or to taste. Heat over very low heat for 20 minutes to give the flavors time to marry, then remove the herbs (and strain the gravy, if desired, for a smoother sauce). Bring the gravy back to a simmer and whisk in the beurre manie, a little at a time, until the gravy is thickened to taste.
Step 6The gravy can be made up to a few hours in advance and kept warm in a thermos.