Use a mixture of cornstarch/water or flour/water to easily thicken beef stew cooked in a slow cooker. For every 1 cup of liquid, use one of these combinations: Cornstarch is a gluten-free option. Mix together one tablespoon of cornstarch with an equal amount of cold water to create a slurry. Mix it into the hot liquid in the slow cooker and stir. It takes about 15 minutes for the sauce to thicken. Flour can be used in the same manner. Combine 2 tablespoons of flour with 1/4 cup of cold water instead. Either way, do not overcook once you've added the thickeners or they will begin to break down, and the sauce will thin out again.