1. Preheat the oven to gas 4, 180°C, fan 160°C. Put the potatoes in a pan of cold water, bring to the boil and simmer for 25 minutes, or until tender.

2. Drain the potatoes and mash with a knob of butter and 50ml of the milk. Stir through the lemon zest, dill and parsley; season to taste. Set aside

3. In a separate pan, melt the butter. Add the flour and cook for 2 minutes, stirring to make a paste. Gradually stir in the remaining milk and the cream to make a thick white sauce; season. Stir in the mustard and chives

4. In a medium pie dish, combine the fish, prawns and white sauce. Top with the mash, then dot with butter. Bake for 30 minutes, or until golden. 

Recipe via Tesco