Finely chop the fresh chervil and blend it with 50g of softened butter. Shape into a roll, wrap, and chill in the fridge until firm.
Wash and poach the langoustines in court-bouillon. Shell them, reserving the tails (heads optional).
Drain the sweetbreads, blanch, and cool. Sauté in butter in an ovenproof dish with sliced carrot, minced onion, and bouquet garni. Deglaze with white wine, add a splash of water, and braise in the oven for 10 minutes.
Once cooked, drain, degrease, remove nerves, trim neatly, and slice into medallions. Strain the cooking juices through cheesecloth and reserve. In a hot pan with a knob of butter, sear the medallions briefly on each side, then pat dry. Add langoustine tails to the same pan, sear quickly, and pat dry.
Arrange medallions on plates, surround with tails.
In the pan, add 2 tablespoons of reserved sweetbread juices, reduce briefly, then whisk in chervil butter. Season with salt and pepper.
Warm plates in the oven and generously coat with the sauce.
Chef's Tip: Thoroughly denerve the sweetbreads—any remaining nerve or skin will compromise the dish's delicate texture and flavor.
