Melt the butter in a heavy-bottomed pot/saucepan over medium-high heat (9 out of 14 on my stove).
Add half of the flour while whisking well. Once it's free of lumps, add the rest of the flour and whisk again.
You will end up with a "paste" which you can let cook for a couple of minutes to ensure the flour is thoroughly cooked.
Add the milk a little at a time, still whisking vigorously. I might add approximately 1.5 dl at a time.
Repeat with the rest of the milk, and after about 10 minutes, you can start to season the sauce. It needs salt, pepper, and grated nutmeg.
Let it simmer for a couple more minutes, and the sauce is ready to serve - or to be made into parsley sauce or used on top of your lasagna.
Enjoy.