For the vegetable garnish: 1 carrot, cut into julienne strips The white part of 1 leek, halved lengthwise, washed well, julienned 2 ribs of celery, cut into julienne strips 1/2 stick (1/4 cup) unsalted butter 1/4 cup all-purpose flour 2 large egg yolks 1/2 cup heavy cream 1/4 cup finely chopped fresh chervil or fresh parsley leaves 1 tablespoon fresh lemon juice, or to taste boiled potatoes as an accompaniment if desired To a kettle of boiling salted water add the whole chicken, return the water to a fast boil, and transfer the chicken to a platter. Discard the water and in the kettle combine the stock, the whole carrot, the halved celery, the halved leeks, the onion, the thyme, the bay leaf, the chicken, and salt and pepper to taste. Bring the stock to a boil and simmer the mixture, covered, for 35 to 40 minutes, or until the juices run clear when the fleshy part of the thigh is pricked. Transfer the chicken to a platter, reserving the cooking liquid, let it cool just until it can be handled, and discard the skin. Transfer the chicken to a tureen and keep it covered. In a large sieve set over a saucepan strain the stock, discard the solids, and keep the stock warm over low heat. Make the vegetable garnish: In a large saucepan of boiling salted water boil the carrot strips for 2 minutes, add the leek strips and the celery strips, and boil the vegetables for 1 minute, or until they are just tender. Drain the vegetables in a sieve, refresh them under cold water, and drain them well. In another large saucepan melt the butter, whisk in the flour, and cook the roux over low heat, stirring, for 3 minutes. Remove the pan from the heat, add the strained stock in a stream, whisking, and bring the mixture to a boil. Boil the mixture, whisking, for 1 minute, reduce the heat, and keep the mixture at a simmer. In a small bowl whisk together the egg yolks and the cream and add 1 cup of the simmering mixture in a stream, whisking. Add the mixture to the remaining simmering mixture, whisking, cook the sauce over moderately low heat, whisking, until it is thickened, but do not let it boil. Stir in the chervil, the lemon juice, and salt and pepper to taste. (Do not let the sauce boil.) Add three fourths of the julienne vegetables to the tureen around the chicken, ladle the sauce over the chicken, and garnish the chicken with the remaining julienne vegetables. Serve with the potatoes if desired. Serves 4 to 6.