Preheat oven to 375 degrees.
Drain tuna; flake.
Break spaghetti into 3-inch pieces.
Cook in salted water until tender; drain.
Saute mushrooms in butter over medium heat for 2 minutes.
Combine mushrooms with spaghetti and tuna in 2 1 /2-quart casse- role.
Blend soup, milk, sherry and nutmeg; stir into spaghetti mixture.
Sprinkle with cheese.
Bake for 35 minutes or until heated through and cheese is lightly browned.