Peel the onions and cut into thin rings.
Peel the garlic and cut into thin slices.
Blanch the tomatoes for a few seconds, peel, quarter, core and dice finely.
Rinse and trim the fennel and cut into thin slices. Set the greenery side.
Sauté the peppercorns, the coriander seeds and the fennel seeds with the garlic, onion and fennel in some olive oil for about 5 minutes. Add the tomatoes, lemon juice, vinegar and wine and season with salt, pepper and sugar.
Lay the trout fillets side by side in a flat dish and drizzle with the hot marinade Cover and place in the refrigerator overnight.
Serve garnished with fennel leaves.