As one of the four "Mother Sauces", I use this quite a bit. Â I use the recipe from the Joy of cook, although it can be modefied in quite a few ways.
Combine in a small saucepan over very low heat:
1 1/4 cups milk
1/4 onion with 1 bay leaf stuck to it using 2 whole cloves
Pinch of freshly grated nutmeg
Simmer gently for 15 minutes, and then discard the onion.