Gruyère and Parmesan Mornay Sauce

Melt. Melt butter over a medium-high flame (don't let it brown) in a little saucepan. Add flour and beat to form a paste. Carry on cooking, mixing, till raw flour scent is gone, around 1 minute. Beating the whole time, add milk in a thin steady trickle, or in increments of a couple of tablespoons after another, beating well plus getting in all corners of the pan in order in order to maintain one homogeneous texture. Sauce will initially become very thick, next get very thin when all the milk is added. Heat, mixing, till sauce comes to a simmer and begins to thicken a little. Reduce heat to low and cook, mixing, till sauce is thick enough in order to coat the back of a wooden spoon, around 3 minutes. Add salt and pepper till it tastes right. Add nutmeg till it tastes right, if using. Simply beat well to take out them plus smooth out sauce, or use an stick blender or jug blender if lumps are particularly big or tough if any lumps form. Working in 2 or 3 additions, beat in cheese, incorporating every addition just as the one before it is upon the verge of fully melting; check that to hold the heat low after this cheese goes in, as too much heat can cause this sauce to break. Take out from heat. Use sauce right away, or push a piece of cling film over surface of sauce in order to prevent a skin off forming plus hold warm till ready to use. Sauce can be cooled plus stored in a closed container in the fridge for several days. Reheat very lightly on this stovetop or in the microwave before use.