In a large heavy skillet over medium high heat, heat the butter until hot. Stir in the onion and mushrooms and cook, stirring occasionally, until browned, about 8 minutes. Stir in the beef tenderloin, 1 teaspoon salt and 1/2 teaspoon pepper, and cook, stirring occasionally, until the beef is browned, 8 to 10 minutes. Stir in the cream and boil until slightly thickened, about 4 minutes. Season the Stroganoff to taste with salt and pepper and serve sprinkled with the dill and spooned over egg noodles dolloped with sour cream.