Method

Heat the oil in a large pan. Add the potato and celery, stirring to coat, then cover and cook gently for 15 minutes until tender, stirring once or twice.

Meanwhile, melt the butter in a pan and remove from the heat. Stir in the flour, return to the heat and cook for 1-2 minutes, stirring. Gradually pour in the low-fat milk, whisking until smooth. Bring to the boil, then reduce the heat and simmer for 2-3 minutes, stirring occasionally. Season lightly with pepper.

Preheat the oven to 180°C/350°F/Gas Mark 4. Spread the onion out in a small roasting tin and arrange the fish on top. Season lightly with pepper and pour over about four tablespoons of the fish stock. Cover loosely with foil and bake for 10 minutes until just opaque. Leave to cool a little, reserving the onion mixture and then break up the fish into bite-sized pieces.

Add the white sauce, reserved onion mixture and the remaining fish stock to the vegetables and gently bring to the boil, stirring well to combine. Blitz with a hand-blender to a smooth puree and then fold in the cooked fish and prawns and simmer for 1-2 minutes to just heat through. Ladle into bowls and garnish with celery leaves to serve.