Boil macaroni until tender, rinse in cold water, and drain.
Melt 1 tablespoon of the butter in skillet and add onion. When onions are translucent, add fresh mushrooms and cook until tender.
In a medium-size casserole dish, combine macaroni, onion and mushroom mixture, tuna, mushroom soup, sour cream, salt and pepper.
Melt remaining butter and mix with bread crumbs. Sprinkle over top of casserole. Top with grated cheese and bake in for about 30 minutes at 350 degrees F.
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