Cook the pasta according to pack instructions, then drain. Heat oven in a fan forced oven 180 degrees, 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 bring to a simmer, stirring for a few mins to thicken. Turn off the heat stir in gravy, stir in the ham and most of the cheese, then season to taste.

Add in the pasta, mashed vegetables, and mushrooms into a large ovenproof dish, then pour over the gravy sauce and mix well in the dish. Scatter over the remaining cheese and spring onions, then bake for 10 mins until golden.