Season the tuna with 1 tsp. salt and 1/4  tsp. pepper. Heat the oil in a 12-inch skillet over medium-high heat. Arrange the tuna in the skillet in a single layer and cook, turning once, until done to your liking (3 to 4 minutes for medium rare). Transfer the tuna to a large plate. Reduce the heat to medium and add the shallot to the skillet. Cook, stirring, until golden-brown, about 1 minute. Add the tomatoes, olives, basil, 1/2 tsp. salt, and a few grinds of pepper; cook until warmed through and the tomatoes are just softened, about 2 minutes more. Remove the skillet from the heat and gently stir in the lemon juice. Transfer the tuna to plates, top with the tomato salad, and serve.