Method

1. To make the sauce, place half the crème fraîche in a blender and add the prepared watercress. Whiz it together on pulse setting of the mixer. Add the remaining crème fraîche and whiz again (still on pulse) until all is blended together well. Do not blend too much as it will turn to cheese. Pour into a container, add salt and pepper to taste, cover and refrigerate until ready to use.
2. For the halibut, butter a suitable ovenproof dish. Scatter two slices of lemon, the lemon balm leaves and sprigs
of parsley in the dish. Season lightly with sea salt and white pepper.
3. Place the fish in the dish in a single layer and spaced apart. Season the fish lightly with salt and pepper. Squeeze a few drops of lemon juice from the remaining piece of lemon on top of each piece of fish.
4. Carefully cover the whole dish with a sheet of foil. Try to avoid the foil touching the fish. Make sure the dish is sealed completely around the edges. Place the dish in the centre of a preheated oven 375F/190C/Gas5. Bake for 15-20 minutes, depending on the thickness of the fillet.
5. To serve, reheat the watercress crème fraîche slowly in a small saucepan. Remove the fish dish from the oven. Carefully remove the foil and spoon some of the cooking juices into a small jug (do not include the herbs etc). Loosely cover the fish with the foil and keep in a warm place.
6. Add the fish juices to the sauce and stir well. When completely warmed through, check for seasoning and consistency. Take care not to let the sauce overheat and split. You may want to add a little cream if the sauce seems too thick.
7. Lift each piece of halibut on to warmed plates and pour the sauce around the fish. Serve with baby new potatoes and fresh mange-touts, sugar snap peas and fine beans, or with a watercress, chicory, orange and fresh herb salad - dressed with a classic vinaigrette.