Arrange tomatoes on a serving platter. Heat a cast-iron grill over high heat until very hot. Quickly sear tuna on each side for 1 minute per side. Remove from heat. Mix the garlic, olive oil, vinegar, red pepper flakes, and olives in a bowl and set aside. Thinly slice tuna. Add to bowl with olive oil mixture and toss until well coated. Spoon over tomatoes, sprinkle lightly with salt, and serve.