Peel and slice onions crosswise in thin slices.
Cook onions in boiling water until tender; drain.
Add cheese and mushrooms to the white sauce.
In a greased casserole dish, place a layer of onions; cover with some of the sauce.
Top with buttered bread crumbs and bake in 350Â°F oven for 1/2 hour.
Can be made the day before.
Excellent with a roast.