For the dressing, place 4 ounces of blue cheese in a small bowl and microwave for 15 seconds, until it begins to melt. Place the mayonnaise, yogurt, warm blue cheese, sherry vinegar, 1/2 teaspoon salt, and 1/4 teaspoon pepper in the bowl of a food processor fitted with the steel blade and process until smooth. Set aside or refrigerate until ready to use.
For the salad, combine the celery, radishes, and scallions in a bowl. Slice the whole head of lettuce across to make four 3/4-inch-thick round disks and place each on a dinner plate. Spoon the dressing on the lettuce and sprinkle on a quarter of the vegetable mixture. Distribute the remaining crumbled blue cheese on the salads, sprinkle with fleur de sel and pepper, and serve.
For taping, Ina prepared the dish on one large platter.