Blanch the crayfish in boiling water for 1-2 minutes, then refresh in cold water. Peel and set aside.
Clean the celeriac, leeks, and potato. Peel the potato, onions, and garlic clove. Wash the tomatoes. Dice the vegetables finely, except the potato.
Crush the crayfish shells lightly, then sauté in a mix of hot butter and sunflower oil. Add celery, leeks, onions, garlic, tomatoes, peppercorns, thyme, bay leaf, dill, and parsley.
Deglaze with Noilly-Prat and white wine, reducing quickly. Add fish stock to cover the shells. Grate raw potato into the mixture and simmer for 30 minutes.
Meanwhile, clean and blanch the small onions, then slice them. Prepare thin matchsticks from carrots, potatoes, and celery.
Season zander fillets with salt and a squeeze of lemon juice.
Pat the vegetables dry, season with salt and pepper, then arrange tightly around the fillets.
Sauté the wrapped fillets in butter until golden on both sides.
Strain the sauce, then whisk in cold butter for a silky finish. Combine with sliced onions and crayfish, and serve under the zander fillets.
Source: The New Kitchen
