Bechamel sauce

I have never had a problem with this béchamel recipe. No clumps, no over-whisking, no stress. The consistency is between thick and thin. It changes quickly though if not used immediately. It thickens and a film is formed on top. If I use the sauce in lasagnas, or anything that would end up in the oven, OK - béchamel can wait. But if I want to serve right away on a plate (goes well with just about anything), I try to avoid reheating. It never seems to regain that desired smoothness. 

I've tried other béchamel recipes (also by Julia Child) that used a bit more flour and added hot milk (liquid). When I added boiling milk, I often ended up with a sauce that was too thick. Giada didn't specify the temperature of the milk so I just heated up mine slightly. I prefer her recipe. The process is smoother. And so is the béchamel.

Recipe adapted from Giada De Laurentiis
time: 5 min preparation + 6 min cooking = 11 minutes
makes about 2 cups

ingredients
2 Tablespoons (30 g) butter
4 teaspoons flour (14 g) flour
1 1/4 cups (300 ml ) warm whole milk
1/4 teaspoon (2 g) salt
1/8 teaspoon freshly ground black pepper
pinch of ground nutmeg

directions
In a medium, heavy bottom skillet or sauce pan, melt butter over medium-low heat (set on 4 on a scale 1-10). Add flour and cook for 3 minutes, whisking continuously (same heat). Whisk in the warm milk. Increase the heat to medium-high (set on 6). Simmer for 3 minutes while whisking the sauce till thickens. If they are clumps, stir more and faster - they should disappear. Whisk in the salt, pepper and nutmeg.