In this simple, healthful salad, the tuna and beans count selenium as antioxidant sources. While the tomatoes have lycopene -- sauteeing them with a little olive oil boosts this antioxidant's bioavailability.
Combine tomatoes, beans, tarragon, lemon juice, scallions, and oil; season with salt and pepper. Set aside.
Heat grill to medium; lightly brush grates with oil. Season tuna, on both sides, with salt and pepper. Grill, turning once, until medium-rare, 2 to 4 minutes. Serve tuna topped with tomato and bean salad.