In a jar with a tight-fitting lid, combine the buttermilk, sour cream, vinegar, oil, dill, and scallions. Shake until well combined; season to taste with salt and freshly ground pepper.
Rinse the lettuce wedges with cold water and shake or pat dry with paper towels. Place a wedge on each of 6 plates, divide the tomatoes among the plates, and drizzle with the dressing. Serve immediately.