In a large (10 to 12-inch), deep saute pan, cook the butter and onion over medium-low heat for 10 to 12 minutes, until the onions are lightly browned—don't shortcut this step as it gives the gravy phenomenal flavor.
Sprinkle the flour over the top, then stir in the salt and pepper. Cook, stirring continuously, for 1 full minute.
Gradually add the turkey drippings, whisking the entire time so that no lumps form.
Add the brandy and sage.
Bring to a simmer and let bubble gently, stirring often, until the gravy thickens to your liking, about 4 to 5 minutes. Carefully taste (it's hot!) and adjust seasoning as desired. Enjoy hot with everything.
Notes
*To de-fat turkey drippings (separate the fat), the easiest way is to use a fat separator like this. That said, it isn't the only option! You can let the drippings rest for several minutes until the fat rises to the top, then suck out the fat with a turkey baster. Or, place a gallon-sized ziptop bag in a large bowl. Carefully pour the drippings into the bag and seal. Let stand for several minutes, until the fat rises to the top. Lift the bag over a cup or bowl, cut a small hole in the corner, and strain the de-fatted liquid into a liquid measuring cup with a spout. Stop pouring when the fat almost reaches the bag opening.
TO STORE: Refrigerate gravy in an airtight storage container for up to 3 days.
TO REHEAT: Gently rewarm leftovers in a saucepan on the stovetop over medium-low heat, stirring frequently. You can also reheat gravy in the microwave.
TO FREEZE: Freeze leftovers in an airtight freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.