Most cooks know it’s possible to thicken a sauce or gravy by adding flour – but it takes a little finesse to do it correctly. If you’ve ever made the mistake of adding flour directly to hot sauce, you know the result is a thin sauce full of flour lumps that won’t go away. Fortunately, dissolving flour before adding it as a thickener will help you get smooth, creamy results every time.
Dissolve the flour by stirring it together with a bit of cool water in a small bowl or cup. Use a fork and stir quickly until you have a very thin slurry the texture of cream. The rule of thumb for figuring quantities is to use about 2 tablespoons of flour to thicken each cup of liquid.
Stir the slurry just before using and pour it slowly into your hot liquid in a thin stream, whisking the dish as you do so.
Bring the dish back to a simmer. This is important because the flour will only reach full thickening power when the cooking liquid is heated to the boiling point.
Monitor the dish for a few minutes and check its consistency. If you would like the liquid to be a bit thicker, add a little more slurry as before. Bear in mind that liquids thicken as they cool, so the dish's consistency on the stove should always be somewhat thicker than what you want at the table.
Taste the sauce and adjust seasonings as needed -- adding flour can sometimes impart a raw flavor to sauces, and also mute tastes, particularly salt.