Difficulty:[usr 3]
Melt the butter in the microwave or over low heat, remove the foam that floats on the surface, keep the thick yellow liquid and discard the whey that remains at the bottom of the container (clarification). Reserve this clarified butter.
Finely grate the celeriac, blanch it for 30 seconds in boiling salted water, then drain and refresh it under cold water.
Prepare a mayonnaise, mix the egg yolk, mustard, salt and pepper, pour the oil in a thin stream until the mayonnaise sets and is very thick. Add the cream and half the grapefruit juice. Thoroughly mix celery and mayonnaise.
Put the clarified butter and the remaining grapefruit juice in a pan. Cook the scallops in this mixture for 3 minutes on one side and 2 minutes on the other. Sprinkle with fleur de sel from Guérande and white pepper.
Serve immediately with the celery in the center and the walnuts around.
Sprinkle with sprigs of chervil.
