Make the sauce: Trim the tough stems from the sorrel and shred the leaves fine, setting aside a little for garnish. Wash the leaves and put them in a saucepan. There will be enough water clinging to the leaves to cook the sorrel. Over medium heat, cook the sorrel for 3 or 4 minutes, stirring constantly, until it is soft and wilted and forms a puree.

In a small saucepan, bring the cream to a boil and let it simmer for 2 or 3 minutes to reduce it slightly. Stir in the sorrel puree and simmer for another minute or two. Strain the sauce, stir in the chives, and season the sauce with some salt and pepper.

Preheat the oven to 450°F.

Season the fish on both sides with salt and pepper. Butter a shallow casserole and lay the fish in it in one layer, skin side down. Dot the fish with butter and spoon on the cream. Bake the fish for 8 minutes, then turn on the broiler and broil it for 4 minutes.

Reheat the sauce and put it in a sauceboat. Serve the fish from the casserole, garnished with some of the reserved finely shredded sorrel, and spoon some sorrel sauce over each serving.