For marinade, in a small mixing bowl stir together water, sherry, soy sauce, ginger root, and garlic. Reserve 2 tablespoons for the dressing. Place meat in a plastic bag set in a large bowl; pour remaining marinade over meat. Close bag and turn to coat meat. Chill for 1 hour, turning once or twice.
Drain meat, discarding marinade. Grill meat, uncovered, directly over medium-hot coals for 10 to 12 minutes or until juices run clear, turning once.
For dressing, in a saucepan stir together reserved 2 tablespoons marinade, hoisin sauce, brown sugar, salad oil, and vinegar; heat to boiling. Stir in sesame oil. Remove from heat.
Thinly slice meat into bite-size strips. In a large salad bowl combine meat, spinach, onion slices, and sesame seed. Pour hot dressing over spinach mixture; toss gently to coat. Serve on salad plates; arrange plum slices around edge. If desired, top each serving with enoki or straw mushrooms. Makes 6 main-dish servings.