For the blue cheese dressing: Combine the sour cream, mayonnaise, blue cheese, buttermilk, hot sauce and lemon juice in a medium bowl. Mix together until well blended. Season with salt and pepper.
For the salad: Cut off about 2 inches of the root end of each romaine heart and discard. Remove 8 outer leaves from each and place them spine-side down on a large serving platter. Chop up the remaining leaves into 1/2-inch-thick pieces and place in a large bowl. Add the tomatoes and blue cheese dressing and toss to coat.
Evenly distribute the dressed salad among the whole romaine leaves. These can be held in the refrigerator for up to 1 hour before serving.