Step 1

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, ½ teaspoon salt, and ¼ 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.

Step 3

For the salad, combine the celery, radishes, and scallions in a bowl. Slice the whole head of lettuce across to make four ¾-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.