Bechamel sauce is made from a white roux (butter and flour) that we mix with milk spiced with white pepper and grated nutmeg. What is white roux? It is a substance created by cooking fat (traditionally butter) but it may also include margarine, oil or wheat flour. Depending on how much we do toast flour, we will get white, blonde or dark roux. Since bechamel sauce is white, we will use hardly toasted roux. This is also known in many countries as white sauce. 
The amount of flour and fat for roux usually is 50%, what is known as TPT; depending on the sauce use we will add more or less roux per liter of milk. This is not a strict rule since it can be varied by adding a bit more fat than flour, depending on tastes, about 60 – 40% for instance. To make bechamel we start by heating the milk in a Magefesa Vitalia Dutch oven. Melt the butter over low heat in a Magefesa Vitalia sauté pan, add the flour and melt well with the help of a spatula or a set of Teflon beaters, let it cook for some minutes and continue stirring to prevent it from turning brown. Add hot milk (¼) to the sauté pan and stir it until well blended. Repeat the process until the whole milk is incorporated. With this way of adding hot milk, the formation of lumps will be avoided. Let the sauce cook, stirring frequently, for about 5 minutes at least, so that flour flavor disappears, season to taste and spice with freshly grated nutmeg.