White sauce is the family of pale savoury sauces made by thickening milk or stock with a white roux — equal parts butter and flour cooked without colouring. Béchamel, the mother sauce, is milk thickened this way and seasoned; add cheese and it becomes Mornay, add onion and clove and it leans toward soubise. The technique is foundational in Western cooking: make the roux, add the liquid gradually to avoid lumps, simmer to cook out the flour and reach the desired consistency. White sauce binds lasagne, coats cauliflower cheese, and underpins countless gratins and croquettes. Its neutrality is its strength — a blank canvas for flavour.