When you cook a roast, whether it's turkey, chicken, beef, or lamb, you should end up with plenty of browned drippings and fat from the roast when it's finished.
The "drippings" are browned juices and fat. All of the flavor a gravy you make from the drippings comes from those browned bits. You need fat as a base for the gravy.
If you have pan juices, but they haven't browned at the bottom of the pan after the roast is done, you can put the empty pan back in the oven. Set the temp to 450 or 500°F and cook until the juices evaporate and begin to bubble and brown at the bottom of the pan.
In this approach to making gravy, we break up the drippings as well as we can with a whisk, but don't worry about any browned bits in the gravy itself. If you want, you can pulse the gravy in a blender to make it smoother, but we never bother. The browned bits are the best part!
Making Gravy with Cornstarch
1 Remove all but 1/4 cup of fat from pan: Remove the roast from the roasting pan. Pour off all but 4 Tbsp of the fat in the pan. If you don't have enough fat in the pan, add butter so that you have at least 4 Tbsp of fat in the pan.
2 Scrape up browned drippings: While the pan is still warm, scrape the bottom of the pan with a metal spatula to loosen any browned drippings that might be stuck to the bottom of the pan.
3 Place the pan on the stovetop on medium heat. If you are using a roasting pan that won't work well on the stovetop, scrape up all of the drippings and fat and put into a large shallow sauté pan.
4 Make cornstarch slurry: Dissolve 4 Tbsp of cornstarch in the minimum amount of water needed to make a thin paste - about 1/2 cup.
5 Pour the cornstarch slurry into the pan with drippings and use a wire whisk to blend into the drippings.
6 Whisk while slowly adding liquid: Stir with a wire whisk until the gravy begins to thicken. As the gravy thickens, slowly add stock, water, milk, or cream, or some combination to the pan (I like to use stock, my mother usually uses water).
Alternate stirring and adding liquid, maintaining the consistency you want, for several minutes (about 5 minutes).
You will probably add about 3 to 4 cups of liquid all together. Taking into consideration the evaporation that is occurring while the gravy is simmering, you will end up with about 2 cups of gravy.
7 Season to taste with salt and pepper.
Making Gravy with Flour
1 Remove all but 1/4 cup of fat from pan: Remove the roast from the pan. Remove excess fat leaving 4 Tbsp of fat plus juices and browned drippings in the pan.
2 Scrape up drippings and place pan on stovetop on medium heat: Use a metal spatula to scrape up any drippings that are sticking to the pan. Place the pan on the stovetop on medium high heat.
3 Sprinkle 4 Tbsp flour onto the drippings. Quickly stir with a wire whisk so that the flour gets incorporated into the drippings. Let the flour brown a bit if you want, before adding liquid in the next step. (You can also start with a slurry of flour and water if you want.)
4 Whisk while slowly adding liquid: Slowly add stock, water, milk, cream, or a combination to the pan, whisking vigorously to dissolve the flour into liquid.
Allow the gravy to simmer and thicken, and continue to slowly add liquid until you have about 2 cups of gravy. (You'll probably need to add 3 to 4 cups of liquid.)
5 Season to taste with salt and pepper. (Always taste first before adding more salt! It may not need it.)