In a separate saucepan, heat the milk with the onion studded with cloves and bay leaf until it is just about to boil, and let it infuse for 30 minutes.
Temper the roux with a little warm milk. Then, add to the rest of the milk.
Add the bouquet garni and season with salt, white pepper, and nutmeg. Cook over medium heat until it reaches the nappé consistence.
Strain through a fine sieve or chinoy into a clean pan. Serve or reserve in a bain-marie.