Think of a slurry as almost the opposite of a roux. A roux is cooked, uses fat, and is added at the beginning of cooking. In comparison, a slurry is uncooked, needs no fat, and is added at the end of cooking.
To make a slurry, just measure out the flour into a small bowl - use one tablespoon to thicken a small amount of sauce or up to four tablespoons for a big bowl of soup. Add a cup or so of the hot cooking broth to the flour and whisk until they're completely combined. This is your slurry.
Next, slowly pour the slurry into the main pot while whisking. This whole process prevents any lumps from forming and helps the slurry to incorporate evenly. Keep stirring the soup or sauce until it has thickened up.
In place of flour, you can also use corn starch, potato starch, or any other starch. If you already used a roux at the beginning of cooking but want to use a slurry to get an even thicker sauce, use a starch other than flour for the slurry.
Heads up: In culinary school, we were advised that using flour for both the roux and the slurry can cause hydrolysis, which is when the starch loses its ability to thicken. But more on that in a future food science post!
(Image: Emma Christensen for the Kitchn)