Prepare the maurice salad dressing by combining mayonnaise, sour cream, white vinegar, Dijon mustard, sugar, dried tarragon, chopped parsley, and lemon juice in a small bowl. Whisk until smooth and well incorporated, then season with salt and pepper to taste. Refrigerate while preparing the salad.
Wash and thoroughly dry all lettuce leaves, tear into bite-sized pieces, and place in a large salad bowl.
Add halved cherry tomatoes, sliced cucumber, black olives, sweet gherkin pickles, and red onion (if using) to the bowl with lettuce.
Gently fold in the chopped fresh basil.
Arrange sliced hard-boiled eggs around the edges of the salad.
Drizzle the prepared maurice dressing over the salad, starting with about half and adding more as needed.
Add crumbled feta cheese on top.
Season with salt and freshly ground black pepper to taste.
For best flavor, let the dressed salad chill for 15-20 minutes before serving to allow flavors to blend together.