In the saucepan melt the butter over low heat. Blend in the flour, and cook slowly, stirring, until the butter and flour froth together for 2 minutes without coloring. This is now a white roux.
Remove roux from heat. As soon as roux has stopped bubbling, pour in all the hot liquid at once. Immediately beat vigorously with a wire whip to blend liquid and roux, gathering in all bits of roux from the inside edges of the pan.
Set saucepan over moderately high heat and stir with the wire whip until the sauce comes to the boil. Boil for 1 minute, stirring.
Remove from heat, and beat in salt and pepper to taste. Sauce is now ready for final flavorings or additions.
(*) If not used immediately, clean sauce off inside edges of pan with a rubber scraper. To prevent a skin from forming on its surface, float a thin film of milk, stock, or melted butter on top. Set aside uncovered, keep it hot over simmering water, refrigerate, or freeze it.
If you follow the preceding directions, you will always obtain a smooth sauce of the correct consistency. But here are some remedial measures in case you need them: If sauce is lumpy If your roux is hot, and your liquid near the boil, you should never have a lumpy sauce. But if there are lumps, force the sauce through a very fine sieve or whirl it in an electric blender. Then simmer it for 5 minutes. If sauce is too thick Bring the sauce to the simmer. Thin it out with milk, cream, or stock, beaten in a tablespoon at a time. If sauce is too thin Either boil it down over moderately high heat, stirring continually with a wooden spoon, until it has reduced to the correct consistency; Or blend half a tablespoon of butter into a paste with half a tablespoon of flour (beurre manié). Off heat, beat the paste into the sauce with a wire whip. Boil for 1 minute, stirring.