Drizzle lemon juice over tuna; sprinkle with pepper. Marinate in refrigerator 15 minutes; discard lemon juice. Prepare grill or broiler; place tuna on grill rack or broiler pan coated with cooking spray; cook 4 minutes on each side or until medium-well. Break tuna into chunks; set aside.
Steam potatoes, covered, 3 minutes. Add green beans, and steam, covered, an additional 8 minutes or until vegetables are crisp-tender; cool.
Combine lettuce and watercress on a large serving platter. Arrange tuna, potatoes, green beans, tomatoes, eggs, and bell pepper over greens. Top with olives, capers, and anchovy fillets. Drizzle Garlic-Basil Vinaigrette over salad.