In a blender or food processor pulse together the yogurt, buttermilk, basil leaves, shallot, and garlic until smooth and pourable. Taste and season as needed with salt and pepper.
Arrange the sliced tomatoes and cherry tomatoes on a platter. Drizzle with olive oil and season with salt and pepper. Pour the buttermilk-basil dressing over the tomatoes and top with the fresh basil leaves. Serve immediately!
Notes
Buttermilk Basil dressing can be made up to three days ahead of time. Store in the refrigerator in a jar with a tight-fitting lid.