For dressing:

Whisk all ingredients in small bowl to blend.

For salad:

Mix oyster sauce, Sherry, tamari soy sauce, minced garlic, and brown sugar in medium bowl. Stir in steak. Marinate steak at least 30 minutes and up to 2 hours, stirring occasionally.

Combine watercress, mizuna, lettuce, basil, cucumber, and tomatoes in large bowl. Add half of dressing and toss to coat. Arrange greens mixture on large rimmed platter.

Heat oil in large nonstick skillet over high heat. Add onion and shallots and saute 30 seconds. Add beef with marinade. saute until beef is brown outside but still pink inside, about 4 minutes. Add pineapple slices and stir until pineapple is heated through, about 1 minute longer. Spoon steak mixture with any pan juices atop dressed greens mixture on platter. Drizzle salad with remaining dressing and serve.