First melt the margarine in a saucepan. Add the flour and stir until it is a mushy mass. Slowly pour in the milk and stir with a whisk until no lumps can be seen. Pour in enough milk until the sauce is creamy but not too stiff.
Add the mustard and spices, stir well and bring to the boil.
Peel the boiled eggs, cut them into slices and add them to the sauce. Mix slowly with the sauce and let it get hot. Sprinkle with parsley.
These are best served with boiled or jacket potatoes