Cut tomatoes into quarters; remove and discard seeds. Pat dry, and brush each side evenly with 2 tablespoons olive oil.
Cook tomato quarters, in batches, in a hot cast-iron skillet over high heat 1 1/2 to 2 minutes on each side or until blackened. Remove from skillet, and cool, reserving juice from skillet.
Toss tomato with reserved juice, basil, and next 4 ingredients in a large bowl. Cover and let stand, stirring occasionally, 1 hour. Garnish, if desired.
Prep: 20 min., Cook: 20 min., Stand: 1 hr.