1. Preheat grill to medium-high heat (350° to 400°). Cook potatoes on well-greased grill grates 12 minutes or until tender. Toss with 2 tablespoons Caper Vinaigrette; set aside.
2. Cook haricots vert in boiling salted water to cover 4 minutes or until crisp-tender. Rinse with cold water; drain. Toss with 2 tablespoons Caper Vinaigrette; set aside.
3. Sprinkle tuna with salt and pepper. Grill on well-greased grill grates 3 minutes on each side (for medium-rare) or to desired degree of doneness. Cut into 1/4-inch-thick slices.
4. Arrange salad greens, potatoes, haricots vert, tomatoes, and next 3 ingredients on 4 serving plates. Top with tuna. Drizzle with remaining Caper Vinaigrette.