First make the roux
Add 50g/2oz of butter to a small saucepan and heat until it is melted then remove from the heat and add 50g/2oz flour. Using a wooden spoon stir it well to combine; then put it back on the heat and cook for couple of minutes, stirring continually.
Remove from heat, add a little milk from 450ml/1 pint and mix well then put the pan back on the heat and slowly add the milk and keep stirring until it is smooth. At this stage it is advisable to switch to a whisk to be assured of a lump free white sauce. When the white sauce is cooked, remove from the heat and season with a pinch of sea salt and a pinch of freshly ground black pepper and add your desired ingredients. Your perfect white sauce is ready.
To make cheese sauce; add 75g/3oz strong cheddar, grated and a tsp of mustard if you wish.
To make parsley sauce; Stir through a handful of parsley, leaves only, very finely chopped.