Step 1In a large saucepan, combine the onion, carrot, celery, turkey pieces, 6 cups water, the bay leaf, parsley and thyme. Bring to a simmer, partially cover and cook for at least 2 hours.
Step 2After cooking, strain the stock into a measuring cup. You'll need about 3 1/2 to 4 cups of stock. Peel the tough skin from the gizzard; chop the gizzard and heart finely. Set aside.
Step 3When the turkey comes out of the oven, remove it to a platter to rest. Place the roasting pan over a burner set to high. Remove any garlic, onions, herbs or aromatics with a slotted spoon and discard. Let the pan sizzle for a minute; add the stock, scraping the pan with a wooden spoon to free any browned bits.
Step 4Pour this mixture into a fat separator or back into the measuring cup and set aside for a couple of minutes to let the fat separate.
Step 5In a saucepan over medium heat, add one-fourth cup skimmed fat or whatever amount of fat you have plus enough melted butter to make about one-fourth cup. Whisk in the flour and let it cook for 1 to 2 minutes, whisking constantly.
Step 6Slowly add 1 cup of stock, whisking constantly, being careful to add as little of the top layer of fat as you can. The sauce will thicken almost immediately. Gradually add more stock, about 1 cup at a time, whisking until the gravy is just thick enough to coat the back of a spoon.
Step 7When all of the stock has been added, season to taste with salt, sage and a couple of grinds of black pepper. Simmer over low heat for 15 minutes to cook out the taste of the raw flour. Occasionally, use a large soup spoon to skim off the skin of protein that forms on the top.
Step 8About 5 minutes before serving, stir in the chopped giblets. Ladle into a warmed gravy boat.
Roasted shallot gravy: Add 4 whole, unpeeled shallots to the roasting pan with the turkey for the last hour of cooking. When the turkey is finished, peel the shallots, chop finely and add to the gravy.
Herb gravy: Mince 1 shallot along with 2 tablespoons chopped parsley and the leaves from 1 sprig of thyme. Whisk this into the thickened gravy and cook 5 minutes. Omit the sage.
Mushroom gravy: Clean and quarter 1 pound mushrooms. Saute in a very hot pan with 2 tablespoons butter. When almost done, sprinkle in 1 minced shallot; add to the thickened gravy. Cook 5 minutes.