Cut in very  little dices half yellow onion and stir fry with a 2  spoons full  of butter till is soft,

Then add flour enough to pick up all the butter (2 spoons full)

While whisking constantly  add 3 cups of whole milk, till it boils, season with salt, a teaspoon of mustard and a little bit of Worcestershire and voila.

After it boils it won't thick anymore... You can add some mozzarella cheese and use it as the cream for a Lasagna... You can add Curry and Chicken and have another fabulous dish. Or you mix in grated Gouda cheese and cooked and diced chicken and wrap it with a Creps and you will have a delicious dinner!