In a large skillet, heat 1 tablespoon oil over medium-high. Season tuna with salt and pepper. Cook, without turning, until golden brown on one side, 3 to 5 minutes. Turn over, and continue to cook until medium-rare, 1 to 2 minutes more. Transfer tuna to a plate, and set aside.
In a medium bowl, toss together tomatoes, onion, lemon juice, and remaining tablespoon oil; season salad with salt and pepper.
Serve tuna on top of tomato-onion salad, with lemon wedges alongside, if desired.