Combine first 7 ingredients; mix well. Shape into 1 1/2-inch balls; brown in oil in a Dutch oven over medium heat. Remove meatballs and drain; reserve drippings. Cook onion in drippings until tender. Add flour; stir constantly.
Combine sour cream and Worcestershire sauce; gradually add to onion mixture, stirring constantly, until thickened. Add mushrooms and meatballs; gently turn to coat. Cover; simmer 30 minutes over low heat. Serve over hot cooked noodles, and sprinkle with poppy seeds.