Beurre manié (pronounced Burr mahn-ee-AY) is a common way of using flour to thicken sauces, stews or soups. The term means kneaded or rubbed butter and derives from the french verb "manier" which means to handle or manipulate.
To make beurre manié, equal parts by volume flour and butter are blended together by hand or using a spoon or fork. Once they are well blended, the beurre manié is dropped in small quantities, about a teaspoon or so at a time, into the hot liquid and stirred. After about the first tablespoonful, allow the sauce to cook enough for thickening to start before adding more. The butter keeps the flour granules from clumping together when they first come into contact with the hot liquid, thus avoiding lumps. As the dissolves, the flour granules are dispersed into the liquid where the starch thickens it.
The amount of flour and butter to use depends on what you are making and how much liquid needs to be thickened. For one cup (250ml) of liquid, a tablespoon (15ml) of flour will make a sauce about the consistency of heavy cream. Two tablespoons (30ml) will make something more like sour cream. As a general practice, I make more than I think I will need and quit adding when the sauce is just a little thinner than I want. Remember that the sauce will thicken a bit more as it continues to cook, and again as it starts to cool down during service.
Many recipes will call for unsalted or sweet butter, but for the amount used, the salt content doesn't really make much of a difference. If you are really worried though, you can reduce the salt called for in the recipe a little. According to the USDA, one tablespoon of salted butter contains 91 milligrams of sodium, which is equivalent to about the amount found in 1/25th of a teaspoon of salt, so for every 3 tablespoons of salted butter used in the beurre manié, reduce the recipe salt by ⅛ teaspoon.