Preheat the oven to 240°C/475°F/Gas Mark 9.
Make the filo pastry at least 24 hours in advance, and allow to rest in the refrigerator until required.
Sweat 115 g/4 oz of the sorrel, lightly seasoned, in the lemon juice and 10 g/¼ oz of the butter for about 2 minutes, then allow to cool. Scale, bone and skin the sea bass, then season and spread the cooked sorrel down one side of each fillet. Divide the pastry into four equal amounts and roll out, stretching each piece as thin as possible, as described in the recipe below. Cover each fillet separately with the pastry, making sure there are no holes. Melt the remaining butter and brush the parcels liberally with it. Bake them on a baking tray in the oven for 10-12 minutes. Baste the pastry with the butter once again after about 4 minutes.
For the sauce, reduce the fish stock and the white wine until thick and syrupy, then pour in the cream and reduce again until it starts to thicken. At the last minute, add the remaining sorrel to the sauce and season.