Butter a 1 1/2-quart baking dish. Preheat the oven to 350°.
In a medium saucepan or saucier, melt the butter over medium heat. Add the mushrooms and cook, stirring, until tender. Stir in the flour and cook, stirring, until smooth and bubbly. Add the milk and continue cooking, stirring, until thickened. Add salt and pepper to taste, along with the nutmeg and dry sherry.
Stir in the spaghetti and salmon. Transfer to the prepeared baking dish.
Combine the bread crumbs and melted butter and toss with the Parmesan cheese until well blended. Sprinkle over the top of the casserole.
Bake for 30 to 40 minutes, until nicely browned and bubbly.
Calories per serving: 388