Preheat the oven to 200C(400F)(Conventional)/180C (356F) (Fan)/Gas 6 (Gas). Add a generous splash of cream and a ladelful of the reserved pasta water to the mushrooms once ready and simmer for 3-5 mins, stirring often. To cook the white sauce, add 35g butter, 35g flour, and 275ml milk. Set over a medium low heat in a saucepan, whisking constantly until just beginning to thicken. Lower the heat and continue whisking for around a minute or so until it is lovely and thick. Add a splash of cream, and continue whisking for around 30s until it thickens again. If it becomes lumpy, take it off the heat and whisk vigurously until you see no more lumps. If you like, add 70-80g cheese and a sprinkling of freshly grated nutmeg whilst it is still hot, but taken off the heat.