1Remove fat from beef. Cut beef with grain into 2-inch strips; cut strips across grain into 1/8-inch slices. (Beef is easier to cut if partially frozen, 30 to 60 minutes.) In glass or plastic bowl, mix sherry, olive oil, 1 tablespoon soy sauce and sugar. Add beef; toss to coat. Cover; refrigerate 30 minutes.
2Heat 10-inch skillet over medium-high heat until 1 or 2 drops of water bubble and skitter when sprinkled on surface. Add half of the beef; stir-fry about 3 minutes or until beef is no longer pink. Remove beef from skillet; drain. Repeat with remaining beef.
3In large bowl, mix beef and onions. Layer tomatoes, mushrooms and lettuce on beef. Cover and refrigerate at least 1 hour until chilled but no longer than 10 hours.
4In tightly covered container, shake all dressing ingredients until well blended. Pour dressing over salad; toss until well coated.