I like to make this with two different colored heirloom tomatoes, mixing Brandywines or Cherokee Purple tomatoes plus a bi-color, like Mr. Stripey, or a green-when-ripe tomato, such as Aunt Ruby's German Green. Kellogg's Breakfast, an almost tropical-tasting tomato that's persimmon in color, would be a fabulous addition as well.
Arrange the tomatoes on a platter, cutting the slices in half if you wish. Sprinkle with the olives and red onion.
In a medium-sized bowl, whisk together the mayonnaise, sour cream, buttermilk, vinegar, salt, and pepper. Spoon some of dressing over the tomatoes and garnish the platter with basil sprigs. Serve the remaining dressing on the side or save for another salad.