Bring 1 liter of water to a boil in a Dutch oven over high heat. Add the fresh crayfish, return to a boil, cook for 50 seconds, then remove them. Reserve the cooking water and rinse the crayfish under cool water.
Detach the tails from the heads of 16 crayfish, reserving 4 whole ones. Shell the 16 tails and set aside.
Heat the olive oil in a sauté pan. Add the crayfish heads, peeled and chopped onion and carrot, bouquet garni, and tomatoes. Sauté until well browned. Deglaze with cognac, then add the white wine. Cook for 5 minutes, pour in the reserved cooking water, and simmer for 15 more minutes. Strain and reduce to 2 glasses of liquid. Reserve.
Season the crayfish tails and scallops with salt and pepper. Melt a knob of butter in a sauté pan; sear the scallops 1 minute per side, then the crayfish tails for the same time. Drain on paper towels. Deglaze with hazelnut vinegar, stir in the reserved jus, cream, and crushed hazelnuts.
Arrange alternating crayfish tails and scallops on warm plates. Garnish with the warmed whole crayfish.
Finish the sauce by whisking in 2 knobs of butter; season with salt and pepper. Spoon over the seafood.
Note: Scallop season doesn't align perfectly with crayfish, so use frozen scallops—they're available year-round.
