Cut potatoes into bite-sized pieces and cook in boiling, salted water until fork-tender. Drain and cool. Boil green beans 5 minutes until cooked but still bright green. To make dressing, pulse garlic, parsley, green onions, tarragon, salt, lemon juice, vinegar, and oil in a blender until smooth. Add more oil if needed for proper consistency. Assemble salad: place lettuce at bottom, then arrange green beans, potatoes, tomato wedges, olives, eggs, and tuna over top. Serve salad, topped with dressing.