Coarsely chopped onion stew in the oil in a saucepan sandwiching. Add grated celery, flour, vegetable stock cube and pour lukewarm water. Boil until the onions soften completely. Remove from heat and stir with a mixer. Thicken with the egg yolks and lemon juice. Sprinkle with finely chopped parsley and celery leaves. Serve chilled.