Preheat oven to 350F. Place mushrooms and onion in a heavy nonstick skillet with a tiny bit of olive oil. Cook slowly until wilted but not browned. Add 1/2 cup of butter. When butter has melted, add flour and cook, stirring constantly, for 5 minutes over very low heat.
Stir in sour cream and heavy cream with a wooden spoon, then add nutmeg and pepper to taste. Continue to cook, uncovered until mixture has thickened. Stir in parsley. In another pan, lightly saute bread crumbs in 1/4 cup of butter. Pour mushroom mixture into a buttered shallow casserole. Sprinkle top with sauteed bread crumbs. Bake until mixture has set and bread crumbs have browned a little more. Makes 6 servings.