method
1. Pour the milk into a pan. Add the onion slice, peppercorns, mace and bay leaf. Bring almost to the boil, remove from the heat, cover and leave to infuse for about 20 minutes. Strain.
2. To make the roux, melt the butter in a pan, stir in the flour and cook, stirring, for 1 minute until cooked but not coloured.
3. Remove from the heat and gradually pour on the milk, whisking constantly. Season lightly with salt, pepper and nutmeg.
4. Return to the heat and cook, stirring constantly, until the sauce is thickened and smooth. Simmer gently for 2 minutes.
