Add butter to the Thermomix bowl. Melt for 2 minutes / 60 degrees / speed 2.
Add the flour to the bowl. Cook for 1 minute / 100 degrees / speed 1 with the simmering basket on the lid in place of the MC.
Scrape down the sides of the bowl and ensure all flour is on the butter and not on the blades. Cook again for 2 minutes / 100 degrees / speed 1.
Add the milk, salt, pepper and nutmeg (if using). Cook for 10 minutes / 100 degrees / speed 3 with the simmering basket on the lid in place of the MC.
Notes
If using the TM6 measuring cup (MC), when cooking at temperatures over 95 degrees, Thermomix recommends using the simmering basket on the lid in place of the MC.