Steam the langoustines for 5-6 minutes.
Remove shells from langoustines, reserve the heads and tails, and discard the rest.
Crush the langoustine heads and set aside.
Fillet the sole and season the fillets with salt.
Poach sole fillets in fish stock and wine for 3-4 minutes in a shallow tray.
Remove fish to a warm place and pour the cooking liquor into a pan.
Add crushed langoustine heads, bouquet garni, and shallot to the liquor.
Simmer the liquid until reduced by two-thirds.
Steam asparagus for 5 minutes, adding samphire for the final minute.
Strain the reduced sauce through a fine sieve into a clean pan.
Whisk butter into the sauce and season with salt and pepper.
Plate the vegetables and top with fish and langoustine tails.
Drizzle with sauce, lemon juice, and chives.