To make the ranch dressing, in a medium bowl, whisk together buttermilk, mayonnaise, sour cream, red onion, dill, parsley, garlic, lemon juice, sugar, and salt and pepper to taste (I like 1/2 teaspoon salt and 1/4 teaspoon pepper). Refrigerate at least 10 minutes to blend flavors. Whisk to recombine before serving.
To assemble the salad, in a large bowl, combine lettuce, endive, tomatoes, radishes, carrots, and cucumbers. Drizzle with salad dressing and toss until evenly coated.
Lettuce: Choose one or more of romaine, iceberg, mixed greens, spinach, arugula, Bibb lettuce, or butter lettuce.
Endive: Belgian endive is a chicory lettuce with white, tightly curled leaves with yellow tips. Omit it if you can't find it.
Grape tomatoes: 1 ½ cup diced tomatoes may be substituted for the grape tomatoes. Seed your tomatoes if desired.
Yield: This recipe makes about 12 cups of salad, enough for 6 generous (2-cup) servings.
Storage: Store leftovers (undressed is best) covered in the refrigerator for up to 4 days.
Make ahead: Wrap washed, dried lettuces rolled in a clean kitchen towel in the refrigerator. Peel and chop any vegetables the day before. The ranch dressing can be prepared up to 3 days in advance (whisk to recombine before serving).