Arrange tomatoes in a single layer in a large shallow dish. In screw-top jar, combine oil, vinegar, onion, shredded or crushed basil, salt, pepper, and garlic. Cover; shake well. Pour dressing over tomatoes. Cover and refrigerate for at least 4 hours or up to 24 hours. Garnish with fresh basil leaves, if desired. Makes 4 to 6 side-dish servings.