Heat the margarine in a saucepan. Once it is liquid, add the flour and stir. Immediately afterwards, whisk in the milk until all the lumps are gone. Now add the egg yolk and whisk it in. Sprinkle in a little salt, then add as much nutmeg and Maggi until you get the desired flavor.
Do not add too much milk at the beginning.
Rather wait until the sauce slowly thickens and then add more. If at the end everything is much too liquid, add some flour. Tip: It takes a moment for the consistency to thicken, so be patient.