Creating rich, velvety, thick sauces in the slow cooker requires a little bit of work toward the end of cooking. The enclosed nature of a slow cooker traps moisture, which results in tender food but a watery sauce. Most thickening methods are suitable for any type of sauce, so the one you choose depends mainly on the ingredients you have on hand and your preferences for texture and flavor.
Flour provides multiple thickening options and works for a variety of sauces, soups and stews. For the simplest method, mix equal parts of flour and hot water together to make a slurry, using 2 tablespoons of slurry for every cup of liquid in the slow cooker. Stir the slurry in and simmer the liquid in the cooker on high for 15 minutes. You can substitute butter for water in the slurry to form a paste, called beure manie, and stir it into the liquid in the same manner to create a creamier sauce. Alternatively, make a roux by heating the flour and butter in a small pan until it's bubbly, and then stir in one cup of the liquid. Stir the roux into the broth and simmer until it thickens.
A Bit of Starch
Starches thicken sauces quickly. Unlike flour, cornstarch won't turn the color of the sauce milky. Mix one part cornstarch with two parts cold water to form a slurry, using 3 tablespoons of slurry per cup of liquid. Stir the slurry into the broth, setting the slow cooker to high for 15 minutes so it simmers and thickens. Arrowroot powder can be substituted for cornstarch, but use 3 tablespoons of slurry for every 2 cups of broth.
Whipping cream and sour cream impart flavor, color and thickening properties to a sauce. They are best used in cream sauces to ensure the flavor of the cream complements the finished dish. Curdling can become an issue, so resist adding cream until the last 30 minutes before serving. Amounts vary depending on other ingredients and desired thickness, so begin with ½ to 1 cup of cream, and combine it with other thickening methods if the results aren't satisfactory. Alternatively, you can whip together 1 egg yolk with 3 tablespoons of cream. Add an equal amount of hot broth from the slow cooker to the mixture, and then stir it into the broth in the last 20 minutes of cooking. Leave the slow cooker on the low setting so the sauce thickens without the cream curdling or the eggs becoming solid.
Slow cooker dishes that contain starchy vegetables, such as potatoes, turnips or carrots, contain their own thickening agents. At the end of the cooking time, remove a few cups of the vegetables and broth and puree them in a blender or food processor before stirring it back in. This method works best for soups, but you can also use it to thicken thinner sauces.
Cooking with the lid off is a slow method for thickening a sauce, but it concentrates the flavor so it results in a richer sauce. To reduce in the slow cooker, turn the cooker to high after the normal cooking time is reached, and remove the lid. You must monitor the sauce so it doesn't reduce too much; the time will vary greatly depending on your slow cooker and the initial volume of the sauce. You can speed it up by transferring the sauce to a pot and reducing it on the stove top, stirring constantly so the sauce doesn't burn.