A smooth, glossy well-flavoured sauce makes all the difference to a simple fish or vegetable dish. Remember that if milk for making a sauce is infused and warmed first, it not only has extra flavour but is much easier to mix in. In a recipe using part hot stock and part cold milk, add the hot liquid to the roux first and stir in the cold liquid afterwards. The addition of a hot or warm liquid keeps the roux soft and makes it a lot easier for mixing. Beat a sauce well as it is coming up to the boil; beating not only makes it smooth but gives the sauce a gloss.