Preheat oven to 180°C (160oC Fan) / Gas Mark 4

For the cheese mash:

Place the potatoes into a large saucepan of cold salted water and bring to a boil. Cook for 15 minutes or until tender, drain well, then return the lid to the pan and allow the potatoes to steam for 5 minutes.

Heat the milk and butter in a small pan, then pour over the potatoes and mash. Sprinkle in half the grated cheese and beat with a wooden spoon until smooth. Season with salt and pepper.

For the sauce:

Heat olive oil in a saucepan and gently fry the onion and leek until soft. Remove to a plate while you make the sauce.

Melt butter in the same pan, then add flour, stirring continuously until a paste forms – this is called a roux. Gradually whisk the milk into the roux until you have a smooth sauce. Cook for 5 minutes, stirring continuously.

Return the onion and leek to the sauce and stir in Dijon mustard, salmon, cod, prawns, peas and season with salt and pepper.

To assemble:

Pour the fish mixture into a pie dish, Top with cheese mash and sprinkle with remaining cheese. Bake in a hot oven for 15 – 20 minutes until bubbling and golden. Serve with fresh green vegetables.