I found out about a quick and less nerve-breaking way to prepare besciamella than the traditional way! That is, al microonde! I recommend it to everybody, for the next time you make lasagne or what ever. For me the end result was without lumps.
250 ml milk
20 g butter
20 g flour
Let the butter melt in a small container in the microwave for 30 seconds, in 800 watts. Mix the flour into it and cook it in the microwave for 20 seconds more. Then pour the milk on it and cook in the microwave for a minute. Take it out. If you find any lumps with a fork, try to destroy them. Add nutmeg and salt, mix and cook for another minute. Mix once again and cook it for another 30 seconds. Use in the cooking you need it in.