1. Heat grill to medium (350° to 450°). In a medium bowl, whisk together oil, vinegar, soy, and pepper; transfer 1/4 cup to a small bowl. Whisk tapenade and parsley into medium bowl of dressing; set aside. Brush steaks, mushrooms, and bell pepper with the 1/4 cup dressing.
2. Grill steaks, mushrooms, pepper, and onion with lid down, turning once, until grill marks appear on pepper and onion, mushrooms are tender, and steak is done the way you like, 8 to 15 minutes total for vegetables and medium-rare meat.
3. Arrange mushrooms, cupped side up, on a platter. Top each with watercress, then a steak, pepper piece, and onion slice. Drizzle with reserved tapenade dressing.