Roux sauce

This is the basis for so many soups, dishes, and side dishes that I've got fed up repeating the basic procedure. I'll post it just the once here and link to it where needed.

Melt a knob of butter in a pan, and mix in a spoonful of flour. You want the resulting mixture to be visibly still buttery, not too sloppy but also not so dry that it's all crumbly. As far as quantities go, it depends what you are making, and whether you want a thin or a thick sauce. For a sauce (e.g. to make a cauliflower cheese) you probably need an ounce or so of butter. More for a soup.

Stir and cook on a low heat for a few minutes. You need to cook the flour but you don't want it to catch on the bottom of the pan or it will burn.

Slowly add milk and whisk out any lumps. If you add a decent splash to start with, the milk should get absorbed by the flour mixture and you end up with a very stiff paste. That's OK, just add more milk and repeat.

The danger is if you add too much liquid too quickly you end up with lumpy milk. To get rid of lumps there needs to be enough resistance in the mixture to whisk them against. If it's too thin, you can recover by passing it through a sieve, rubbing the lumps out with the back of a spoon.

As you add liquid, turn up the heat and keep stirring as it comes to the boil. The sauce will thicken and you add more liquid until you reach the consistency you want.