1. Melt the margarine in a saucepan over a medium heat but don't let it burn.
2. Add the flour and stir until fully mixed together into a thick paste - this forms the "roux". Cook the roux for 1 minute.
3. Gradually add the milk, stirring until fully combined with each addition.
4. When all the milk has been added stir continuously over a medium heat until the sauce begins to thicken.
5. Cook over a high heat until the sauce starts to boil, then reduce the heat and simmer gently for 5 minutes stirring continuously. The sauce should be smooth and glossy without any lumps.
6. Season with black pepper.
40g (1½ oz) plain flour / wholemeal flour
40g (1½ oz) margarine
275ml (¼ pint) semi-skimmed milk (thick sauce) OR
425ml (½ pint) semi-skimmed milk (medium thick sauce) OR
570ml (1 pint) semi-skimmed milk (thin sauce)