If you like a thicker sauce, mix equal parts softened butter and flour into a paste, then whisk into the sauce. Cook for a minute, just to get the raw flour taste out. Or for a more complex flavor, make a light brown roux. Again, equal parts butter and flour, but cooked in a pan, constantly stirring until the desired color is achived. Add stock/sauce gradually to it, whisking after each addition. All the best, Al.