Potato flakes thicken effectively and instantly -- no need to cook out any lingering starch as you do with roux. However, potato flakes -- basically dried mashed potatoes -- impart a textured mouth-feel, so they work best with rustic, country-style sauces, like beef gravy. The main difference in using potato flakes instead of other thickeners is seasoning, because you have to use a little more salt to compensate for the extra body they add.
Adding Potato Flakes
There isn't a specific proportion of potato flakes to gravy to add when thickening, so you have to rely on personal preference. Bring the beef stock to a simmer, but don't season it yet. Add 1 tablespoon of potato flakes at a time to the stock and stir to incorporate. Cook for a few seconds and taste to check the consistency. Continue adding potato flakes a little at a time until it reaches the desired thickness. Usually 1 tablespoon per cup of stock makes a gravy with medium thickness, but it may vary, depending on the coarseness of the flakes.
Seasoning the Gravy After Thickening
Set the heat on the stove to low after you thicken the gravy. Season the gravy with a pinch or two of kosher salt per cup of gravy to start and stir to incorporate. Taste the gravy and then add more salt, a pinch at a time, until it reaches the desired taste. If you need to add other seasonings, such as black pepper or herbs, do so after adding the salt.