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.