Bechamel Sauce (Besciamella)
Put the butter (or oil) in a non-stick pot, and melt the butter while keeping the heat low. Turn off the heat and add the flour a little at a time mixing with a wooden spoon. Turn the heat back on, and add the hot milk a little at a time always mixing. If you don't proceed correctly (for example, the temperature is too high), lumps will form in the mixture. Keep mixing and simmer for at least fifteen minutes. It will get more and more creamy.
Add a pinch of salt and nutmeg; have it tasted by somebody who grew up in Italy to see if it's ready. When ready, turn off the heat and mix in the grated Parmigiano Reggiano.