Melt the butter in a saucepan and stir in the flour with a whisk. Gradually stir in the cream and milk, making sure that no lumps form. When everything is warmed up, add the Parmesan, season lightly with salt and season to taste with pepper and nutmeg.
Bring salt water to the boil, cook tortellini (ready-made or homemade) in it, then drain and add to the heated sauce.
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