Blend the Base: In a food processor, combine scallions, basil, lemon juice, Dijon mustard, olive oil, garlic, salt, and pepper. Blend until smooth (about 15-20 seconds).
Mix the Creamy Ingredients: Add the mayonnaise, Greek yogurt, and buttermilk to the food processor. Blend until the mixture is smooth and creamy.
Chill: Transfer the dressing to a container, cover it, and refrigerate for at least one hour. This allows the flavors to meld together for the best taste.
Serve: Drizzle over Bibb lettuce, tomatoes, and onion slices for a refreshing salad. Alternatively, use it as a dip for fresh vegetables or a dressing for coleslaw.