Trim fat from beef steak; cut beef with grain into 2-inch strips. Cut strips diagonally across grain into 1/4-inch slices. (For ease in cutting, partially freeze beef, about 1-1/2 hours.).
Heat margarine in 10-inch nonstick skillet over medium heat; Cook onion and garlic, stirring occasionally, until onion is softened. Stir in beef and mushrooms. Cook about 7 minutes, stirring occacionally, until beef is no longer pink. Stir in wine; reduce heat. Cover and simmer 10 minutes.
Mix cornstarch and broth; stir into beef mixture. Cook over medium-high heat, stirring occasionally, until thickened; remove from heat. Stir in pepper and yogurt; reduce heat. Cover and simmer about 30 minutes, stirring occasionally, until beef is tender.