Fillet and skin the John Dory. Each fillet breaks naturally into 3 pieces, then split each piece in half lengthwise, so you have 12 strips.
Reduce the fish stock and white wine in a pan until about only 2 tablespoons remain, then add the cream. Split the vanilla pods in two lengthwise, scrape out the centre and add to the sauce. Cut the pods again in two and add to the sauce. Reduce this a little until it just starts to thicken.
Heat the oil in a frying pan and add the butter. Season the strips of fish and fry gently, turning only once, until cooked and slightly browned. The fish should take no more than 1 minute each side to cook. Season the sauce using the lemon juice and the sugar. Remove the vanilla pods from the sauce and reserve.