Grease a large baking dish of at least two litres in size.
Place the new potatoes in a large pan of salted water and bring to the boil and cook until soft – around 20 minutes.
Melt 50g butter in a pan and fry the leeks until softened. Add in the flour and cook for a minute or two before removing from the heat.
Gradually whisk in the hot milk then return to the heat and bring to the boil stirring until thick and smooth.
Stir in the lemon zest, season with salt and pepper and then add in the chopped dill.
Add the fish and prawns and stir through. Cook for a few minutes before pouring into the large baking dish. Drain the potatoes then add back into the pan. Using a potato masher gently press down on the potatoes to break them slightly but not mash them completely, creating rough edges.
Pour in two tablespoons of olive oil and season with salt and pepper then stir.
Place the potatoes on top of the fish mixture then drizzle with a little more olive oil.
Bake in the oven for 40 minutes until the potatoes are crispy and golden and the pie is bubbly.
Remove from the oven and scatter over the chopped parsley.Serve with lemon wedges.