Meat Loaf Stroganoff

1 cup finely chopped onion
1 cup mushrooms, stems closely trimmed, thinly sliced
1/4 cup (1/2 stick) unsalted butter
1/4 cup sherry or dry red wine
2 large eggs
2 pounds ground sirloin
2 cups fine dry bread crumbs
1/2 cup finely chopped parsley
1 cup sour cream
1/2 teaspoon nutmeg

Saut