Place mushrooms, carrots, and leeks in a 3-1/2- or 4-quart slow cooker. Place beef over vegetables. Sprinkle with lemon peel, pepper, rosemary, thyme, and salt. Add broth.
Cover and cook on low-heat setting for 7 to 8 hours or on high-heat setting for 3 1/2 to 4 hours.
Use a slotted spoon to transfer meat and vegetables to serving dish. Cover with foil to keep warm. Skim fat from remaining cooking liquid. Measure 1 cup cooking liquid. Place in a small saucepan. In a small bowl stir together sour cream and flour. Stir into cooking liquid using a whisk. Cook and stir over medium heat until slightly thickened and bubbly; cook and stir for 1 minute more. Ladle sauce over meat and vegetables. Makes 6 servings.