Rather than using a traditional roux of butter or margarine mixed with flour to thicken a sauce, try one or two spoonfuls of pureed vegetable added to a little cooking liquid (or stock). It will thicken your sauces very nicely. If it is more convenient, the liquid and vegetables can be pureed together. Herbs and spices may be added just before serving.

Sweet Turnip Puree Recipe
Healthy Low Carb Diabetic Diet