@thediana I see you're a homecook as well. I made lots of béchamel and derivates in my life and I'd like to share how. Making a béchamel isn't all that tricky.
It is already mentioned here that weight is probably the right thing to. I believe so too, simply because common sense tells it's quite hard to make equal volumes without melting the butter first... however, for a homecook, after a few try-outs you'll be able to make a béchamel -using volumes- without measuring anything, just by eye, or as the French say "au pif".
It's also been said that the ratio butter/flour isn't critical at all. In fact, you need to be able to "play" with the ingredients for the best result.
Making béchamel, using around half a liter of milk (1 lb in weight).
A. Make a roux first with butter and flour.
- Add 2 heaped tbsp. of butter (or margarine!) to a pot and let it melt on medium low flame. Do not let it brown, even better, you can add the flour even before the butter is entirely melted; I always do that. You need a "blond" roux for béchamel.
- Add 2 slightly heaped tbsp. of flour to the butter and let it integrate into the butter while slowly whisking (do not use a wooden spoon for this!); 1. this breaks up lumps, 2. this will give you an idea how the roux evolves. Do make sure to reach all "corners" of your pan, you do not want roux to sediment there or you will get lumps. Using a conical pan is a very good idea.
- After simmering for a while and whisking it slowly, you should have a rather thick pasty looking roux, like the consistency of mayo. If it gets crumbly, then add another 1/2 tbsp. of butter and stir again. If it remains quite thin and not pasty, add a small 1/2 tbsp. of flour and continu to stir until the roux is ready.
B. For the béchamel;
Please, do use cold milk, this will avoid lumps! Also, it works easier for a better result.
- Add a good dash (maybe 1/2 a cup or so) of cold milk to your roux (still on low fire). Whisk vigourously using a whisk. Almost immediately you will get a thick result. Add another good dash of milk, a bit more than the first time. Whisk again. Keep adding milk bit by bit while whisking until the béchamel has the required thickness.
Note; Each time you add milk, it takes a few moments before the preparations thickens, so don't add the milk too soon, give it a bit of time each time you added milk and keep whisking.
- When your béchamel has the right thickness, let it simmer for a good 10 minutes on very low fire.
Italians happen to add a dash of cream at the very last moment (don't boil anymore when doing this), it's the secret of their lasagna (don't tell anyone!).
When you change the milk with stock (veggie, chicken..), either partial or entirely, you have what is called a "velouté".
You can make a béchamel as thick as you want. When made for use it in ovendishes, you might want to leave it thicker... again, play with it.
Another note; Nowadays, in culinary schools, they teach to add cold milk to hot roux or cold roux to hot milk.