Preparation time: 30 minutes
Cooking time: 1 hour

1. Clean the chicken and season the cavity with salt and pepper. Brush the black truffles; cut one truffle into thin slices and insert three slices under the skin of each breast, as well as three other slices into the fat part of each thigh and drumstick. Truss the chicken and set aside.
2. Wash all the vegetables, peel the carrots and the turnips, keeping one centimetre of stem on them.
3. Prepare the young leeks and the new onions, and cut twelve 2 cm long sticks from the celeriac and turn them into nice ovals.
4. Bring the 3 litres of chicken stock to the boil in a pot large enough to hold the chicken; add the bouquet garni. Put in the chicken. Return to a boil and let poach, covered at a temperature of 80° C for 50 to 55 minutes. After 15 minutes of cooking, add the celery, carrots and turnips. After 20 minutes, add the leeks and the new onions.

Making the Mousseline

1. Place the egg yolk into a small pan and whip it into a sabayon over simmering water with 100 ml of reduced cooking liquid from the chicken, as well as the 50 ml of truffle juice.
2. Add the warm clarified butter; thin the sabayon with a little crème fraîche.
3. Take half of the black truffle and cut into small irregular pieces; sauté them very briefly in a pan in butter heated til foamy, season with salt and pepper and fold into the mousseline.
4. Correct the seasoning of the mousseline with salt, pepper and two drops of lemon juice.
5. Keep warm over a bain marie at 40° C.

Serving

1. Place the chicken in a large casserole, pour the broth and all the vegetables over top and remove the bouquet garni.
2. Mince the rest of the truffle, sweat in butter heated to the foaming point, season with salt and pepper.
3. Place the truffles on the chicken breast and serve the chicken like a pot-au-feu: the broth separately in soup cups; serve the truffle mousseline sauce in a sauce boat.