Put the arugula in a large bowl, cover with a damp paper towel, and refrigerate.
In a small bowl, whisk the balsamic vinegar, lemon juice, garlic, and 1/2 cup oil. Season with salt and pepper.
Heat a 12-inch heavy skillet over mediumigh heat. Brush the tuna generously with the remaining 1 to 2 Tbs. oil and season with salt and pepper. Cook the fish until the first side is browned, about 3 minutes. Flip and continue to cook until the second side is browned and the tuna is cooked to your liking, about 3 minutes. more for medium rare. Transfer to a plate and keep warm.
In a medium bowl, combine the beans, tomatoes, olives, and lemon zest and toss with just enough of the dressing to coat, about 3 Tbs. Season generously with salt and pepper. Toss the arugula with just enough dressing to coat lightly, about 5 Tbs., and season with salt and pepper. Portion the arugula onto four plates and top with a mound of the bean mixture. Slice the tuna into 1/4-inch slices and arrange on the arugula around the beans. Drizzle the tuna with a little of the remaining dressing and serve.