How to Make It
Stir together tomatoes, olives, 2 tablespoons basil, 2 tablespoons oil, vinegar, parsley, shallot, capers, and 1/4 teaspoon salt in a bowl.
Heat a grill pan or cast-iron skillet over high. Coat pan and tuna with cooking spray. Sprinkle tuna with 1/2 teaspoon pepper. Grill tuna to desired degree of doneness, about 2 minutes per side for medium-rare. Let stand 1 minute. Cut against the grain into 1/2-inch-thick slices. Sprinkle with remaining 1/4 teaspoon salt.
Toss arugula in a bowl with lemon juice, remaining 2 tablespoons oil, and remaining 1/4 teaspoon pepper.
Top tuna steaks evenly with salsa, and sprinkle with remaining 2 tablespoons basil. Serve with salad.