Cut each tomato crosswise into twelve one-half-inch-thick slices. Cut the mozzarella crosswise into twelve one-half-inch-thick slices. Season both sides of each slice of tomato and mozzarella with the salt and pepper.
Arrange half of the tomato slices in the bottom of a shallow plastic or glass bowl. Top each with 2 basil leaves. Place a slice of mozzarella on top of the basil leaves, then top with the remaining tomato slices and repeat the process using up the remaining mozzarella and basil. Drizzle with the olive oil, cover, and refrigerate for at least 2 hours before serving.
Makes 6 servings