Add the butter to the Thermomix bowl and melt for 3 min/60°C/speed 2. Scrape down the sides of the bowl with a spatula.
Add the flour and cook for 1 min/100°C/speed 1. Scrape down the sides of the bowl with a spatula and along the blades. Cook again for 1 min/100°C/speed 1.
Add the milk and nutmeg and cook for 6 min/90°C/speed 4. Season with salt and pepper.
Serve immediately, or see notes below on storage.
Notes
Storage: Store the béchamel in an airtight container in the fridge for up to 5 days. Reheat gently on the stovetop, stirring frequently.To freeze: Freeze for up to 3 months in a freezer-safe container. Thaw in the fridge overnight. It may look slightly separated when reheated but will come back together.