Cook the noodles in boiling salted water for 10 minutes. Strain and put into a greased casserole.
Slice the mushrooms and the onion and saute in butter until tender. Add to the noodles.
Mix in most of the cheese, reserving some for sprinkling over the top.
Beat the egg, and stir into remaining butter the mushrooms were sauteed in, adding a little boiling water to make a sauce. Pour the sauce over the noodles and mushrooms. Sprinkle with cheese and breadcrumbs and dot with small nobs of butter.