1. Using butter and flour, make a roux. Keep it aside to cool.
2. Boil milk. If you want thinner sauce, use more.
3. Add thyme and nutmeg to it. Remove thyme after a while.
4. Reheat milk and pour it gradually on to the roux.
5. Return to heat. Remember to beat well in between each addition.
6. Add salt and pepper as per taste.
7. Once the sauce attains a smooth consistency, let it stand over hot water. Cover with a wet circle of greaseproof paper to prevent a crust from forming on top. Since this paper lid can prevent crust from forming, this sauce can be prepared well in advance.
8. Bechamel sauce is a main ingredient for many lasagna recipes.