Fish pie with shoestring potato topping

Using spiralized potato instead of mash gives the classic fish pie a crispy topping. An ideal, filling family supper packed full of king prawns

200g frozen peas
3-4 large Maris Piper potatoes (about 600g) peeled, ends trimmed and spiralized into thin noodles
25g butter
25g flour
400ml milk
zest and juice of 1 lemon, plus 1 lemon cut into wedges to serve
1 tsp Dijon mustard
half a small pack of chives, chopped
100g asparagus tips, cut into thirds
300g skinless, boneless white fish cut into large chunks
150g pack raw king prawns
2 tbsp olive oil

Heat oven to 200C/180C fan/gas 6. Bring a large pan of salted water to the boil. Put the peas in a small sieve that will fit the pan. Lower and cook for 30 seconds, then lift out, run under cold water and leave to drain. Drop the potatoes into the boiling water and cook for 1 min to soften partially, then drain. Pat dry with kitchen paper and transfer to a separate bowl.
To make the white sauce, melt the butter in a large saucepan. Add the flour and cook for 1 min. Remove from the heat and slowly whisk in the milk until smooth then return to the heat, slowly bring up to the boil, whisking, then cook for 5–6 mins until you have a very thick sauce. Take off the heat and stir in the lemon zest and juice, mustard and chives. Gently stir in the peas, asparagus and fish.
Transfer the filling to an ovenproof dish. Toss the potato with the olive oil, some flaky sea salt and black pepper. Top the fish pie mix with the potato spirals and bake in the oven for 20–25 mins until hot through. For a crispy topping, put the pie under the grill for 5 mins until the potato is golden.