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Cook the pasta according to pack instructions, then drain. Heat oven to 200C/fan 180C/gas 6, then melt a little butter in a large saucepan. Fry the mushrooms for a couple of mins, then scoop out and set aside. Use some kitchen paper to wipe out the pan.
Melt the remaining butter in the pan, then add most of the onions and soften for 1 min. Stir in the flour for another min, then gradually stir in the milk until you have a lump-free sauce. Increase the heat and bubble the sauce, stirring for a few mins to thicken. Turn off the heat, stir in the ham and most of the cheese, then season to taste.
Tip the pasta and mushrooms into a large ovenproof dish, then pour over the sauce and mix well in the dish. Scatter over the remaining cheese and spring onions, then bake for 10 mins until golden.