Place the scallions, basil, lemon juice, mustard, olive oil, garlic, salt, and pepper in the bowl of a food processor fitted with the steel blade. Puree for 15 to 20 seconds to make a smooth mixture. Add the mayonnaise, yogurt, and buttermilk and blend until smooth. Transfer the dressing to a container, cover, and refrigerate for 1 hour for the flavors to develop.
Arrange the lettuce, tomatoes, and onion artfully on salad plates and drizzle with the dressing. Sprinkle with salt and pepper and serve.
Cook's Note: Ina substituted Boston lettuce for Bibb lettuce when taping this episode.
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