Béchamel sauce, also known as white sauce in English, is very common in French cuisine, and the base for many other sauces.
For 400 grams of sauce:
In a saucepan, make a roux by melting 100 grams of butter over a medium heat.
Mix in 30 grams of flour in the melted butter, until the mixture makes a thick paste. Cook the paste until bubble start forming.
Slowly incorporate around 300 millilitres of milk, and gently bring the sauce to a boil. You can vary the quantity of milk until it reaches your preferred consistency.
Once cooked, add a pinch of salt.