Method
Make a roux with the butter and flour and allow to cool. Boil the milk, using more for a thinner sauce. Add the thyme, nutmeg, and remove thyme. Reheat the milk and pour gradually on to the roux, returning to heat and beating well between each addition. When the sauce is smooth, stand over hot water and cover with a wetted circle of greaseproof paper so that a crust will not form on the top. With this paper llid to prevent a crust forming this sauce can be prepared well in advance.