Put 1 liter of water in a Dutch oven over high heat. Bring to a boil. Throw in the castrated crayfish. Let boil again, count to 50 and remove the crayfish. Save the cooking water. Rinse the crawfish under cool water.
Separate the tails from the heads of 16 crayfish. Reserve the whole four. Shell the 16 tails. Reserve them.
In a sauté pan, put the olive oil. Add the crayfish heads, onion and carrot, peeled, washed and cut into pieces, the bouquet garni. Fry everything and the tomatoes well. Deglaze with the cognac and moisten with the white wine. Cook for 5 minutes and add the reserved cooking water. Leave to cook for another 15 minutes. Then filter this juice and reduce it again until you obtain 2 glasses of liquid. Book.
Season the crayfish tails and scallops with salt and pepper. Put a knob of butter in a sauté pan, brown the scallops for 1 minute on each side, then count the same time for the crayfish. Reserve on absorbent paper. Deglaze with hazelnut vinegar. Add the crayfish juice, then the cream and the crushed hazelnuts.
Arrange the crayfish by alternating them on plates with the scallops. Decorate with whole crayfish and warm.
Winnow the sauce with 2 knobs of butter, salt and pepper. Take the plates out of the oven and coat.
Note:The scallop season does not coincide exactly with the crayfish season, so I advise you to use frozen scallops, although scallops are now available practically all year round.
