1. If the lettuce is not prewashed, rinse it, drain, and spin or pat dry. Place a lettuce leaf on each serving plate.
2. Combine the mayonnaise, sour cream, lemon juice, salt, and pepper in a 2-1/2-quart or larger bowl, and whisk to blend well.
3. Cut the turkey into bite-size chunks and add to the bowl. Using a small serrated knife, cut the grapes in half and add them to the bowl. Cut the scallions into thin slices, including enough tender green tops to make 1/4 cup, and add to the bowl. Stir gently but thoroughly to mix with the dressing.
4. Top each lettuce leaf with 1 cup of salad, garnish with 2 tablespoons pecans, and serve.