1. Peel the celeriac, cut it into cubes, and cook it in boiling salted water for 20 minutes until tender. Drain, blend with the cream and 20 g of butter. Season with salt and white pepper. Keep warm.
2. Sear the chicken breasts skin-side down in a hot pan with oil until nicely browned. Turn over, add the lemon thyme and a little butter. Baste regularly. Finish cooking for 8 minutes in the oven at 350°F.
3. Deglaze the pan with the chicken stock, reduce over low heat to obtain a rich, shiny sauce.
4. Arrange a bed of mousseline, place the sliced chicken on top, coat with the reduced sauce, and sprinkle with lemon thyme leaves.
