Preheat the oven to 175 degrees. Wash the fillets, pat dry and place them side by side in an ovenproof dish. Drizzle with lemon juice, salt and pepper. Mix the cream, fish stock and herbs, season and pour over the fish. Butter a large piece of aluminum foil. Place the buttered side on the fish and use it to seal the pan tightly.
Cook the fish in the oven at 175 degrees (gas: level 2) for about 15 minutes. Arrange on preheated plates. Thicken the cream sauce with cornstarch as desired, season to taste and serve with the fish.