Start by preheating your oven to 350°F (180°C). In a saucepan, melt half of the butter over medium heat, then add the onion and mushrooms. Cook them gently, stirring occasionally, for about 5 minutes until they’re softened and fragrant.
Take each fish fillet and roll it up carefully with the skinned side facing inward to create neat spirals. Arrange these rolled fillets upright in a shallow baking dish, then spoon the cooked mushroom and onion mixture into the center of each roll.
Pour the wine around the fish rolls and sprinkle the tarragon over everything. Season generously with salt and pepper to taste. Cover the baking dish with a lid or aluminum foil and place it in the preheated oven. Bake for approximately 15 minutes, or until the fish turns opaque and the flesh flakes easily when tested.
Once cooked, carefully remove the fish rolls from the baking dish and transfer them to a warmed serving platter to keep them hot. Pour all the cooking juices from the dish into a saucepan and bring to a vigorous boil. Let it bubble away for about 3 minutes until the liquid has reduced by half and intensified in flavor. Stir in the cream and lemon juice, warming everything through gently without boiling. Taste and adjust the seasoning if needed, then pour this silky sauce over the fish. Finish with a garnish of fresh tarragon sprigs before serving.
