Pat the trout fillets dry and season both sides with the salt. Dredge in the flour, shaking off the excess.

Heat 3 tbsp / 45 ml of the olive oil in a 12-inch / 30 cm skillet over medium heat. Fry the fillets in batches so they aren't crowded, skin side down for 3 to 4 minutes, then flip and cook 2 to 3 minutes more, until golden and the fish flakes easily at 145F/63C.

Transfer the fried fillets to a shallow ceramic dish, arranging them in a single layer.

Add the remaining 2 tbsp / 30 ml olive oil to the same skillet. Add the sliced onion and cook over medium-low heat for 8 to 10 minutes, stirring occasionally, until soft and just turning gold at the edges.

Stir in the garlic, sage leaves, bay leaves, and peppercorns. Cook for 1 minute until fragrant.

Pour in the vinegar, wine, water, and sugar. Season with a pinch of salt, bring to a simmer, and cook for 4 to 5 minutes, until the onion is tender but still holds its shape and the sharp vinegar smell softens.

Pour the hot marinade and onions directly over the fried fish, making sure the fillets are fully covered.

Let the dish cool to room temperature, uncovered, for about 30 minutes.

Cover and refrigerate for at least 12 hours, ideally 24, so the vinegar mellows and settles into the fish.

Serve cold or at room temperature, spooning extra onion and marinade over each fillet.