This isn’t even a recipe but kind of an essential kitchen know-how as it is used in so many sauces and dishes. It is basically equal measures of butter and flour cooked together in a pot for a few minutes and is used to thicken sauces.
Melt 100g butter in a pan and stir in 100g of plain flour. Whisk together until they form a paste. Best to continue to stir for 3-4 minutes to allow the flour cook out a little.
Cover and leave in the fridge in an airtight container and it will keep for several weeks.
Just grate in little knobs of it as you need for your white sauce, parsley sauce, cheesy sauce and so on. 15-30grams per dish should suffice but I would just add in by eye.