Whisk the balsamic vinegar, lemon juice, minced garlic, and all but one tablespoon of the olive oil together in a small bowl until well combined.
Season the tuna steaks lightly with salt and pepper, then brush them with the remaining tablespoon of olive oil.
Heat a skillet over medium-high heat and sear the tuna for about 3 minutes per side until cooked to your preferred doneness.
Combine the beans, tomatoes, olives, and lemon zest in a large mixing bowl, then toss with 3 tablespoons of the prepared dressing.
Add the arugula and another 5 tablespoons of dressing to the bowl, tossing gently until the greens are lightly coated.
Divide the salad among four plates and top each portion with a sliced tuna steak.