Yellow-red Mr. Stripey tomatoes give this ice a light hue. Use red or purple tomatoes if you want a darker granita, if you prefer. Mixing purple basil with deep-green Italian basil yields a nuanced flavor and a lovely garnish.
To prepare granita, place vinegar, oil, 1/4 teaspoon salt, and seeded peeled tomato in a blender or food processor; process until smooth. Place tomato mixture in an 8-inch square baking dish; cover and freeze until firm, stirring twice during first 2 hours. Remove mixture from freezer; scrape entire mixture with a fork until fluffy.
To prepare salad, arrange tomato slices on a platter. Sprinkle tomatoes with pepper and 1/4 teaspoon salt. Top with granita. Sprinkle with basil.