Gravy is typically made by using a thickener such as flour or cornstarch that works with the fats produced when roasting meat or poultry. While it takes a little longer, the same effect can be achieved by using the reduction method, in which the principle of evaporation replaces the action of a thickening agent. As the liquid simmers, it releases moisture in the form of steam, leaving fat solids, bits of meat, and concentrated flavorings behind. The amount of time you allow the reduction to simmer determines how thick the final gravy will be.
Use the roasting pan that contains the meat drippings or transfer the drippings into a smaller saucepan. Place either the roasting pan or the saucepan over medium heat. Add a pat or two of butter to increase the fat content, if desired, and stir the fat and the bits of roasted meat until blended.
Add finely chopped seasoning vegetables such as garlic or onions, if desired. Cook and stir them in the drippings for about two minutes, or until just tender.
Add about 1 1/2 cups water, broth, wine or a combination of the three to the pan drippings, and scrape the bits of roasted meat from the bottom of the pan to incorporate them into the liquid.
Bring the pan drippings and the water to a boil, stirring constantly. Reduce the heat to low and add any seasonings, including salt and pepper, to your taste. Do not cover the pan, and allow the mixture to simmer gently to allow the moisture to evaporate. Whisk the gravy as often as needed to eliminate any lumps that form during cooking.
Remove the pan from the heat when you notice that the residue line on the inside has dropped from the original starting point to about halfway from the bottom, as this indicates that the gravy has been reduced by about half.