Heat the oven to 200C/fan 180C/gas mark 6.
Put 1kg potatoes, peeled and halved, in a saucepan and pour over enough water to cover them. Bring to the boil and then simmer until tender.
When cooked, drain thoroughly and mash with a splash of milk and a knob of butter. Season with ground black pepper.
Put 25g butter, 25g plain flour and 4 finely sliced spring onions in another pan and heat gently until the butter has melted, stirring regularly. Cook for 1-2 mins.
Gradually whisk in 400ml milk using a balloon whisk if you have one. Bring to the boil, stirring to avoid any lumps and sticking at the bottom of the pan. Cook for 3-4 mins until thickened.
Take off the heat and stir in 320g-400g mixed fish, 1 tsp Dijon or English mustard, a small bunch of finely snipped chives, handful of sweetcorn and handful of petits pois. Spoon into an ovenproof dish or 6-8 ramekins.
Spoon the potato on top and sprinkle with a handful of grated cheddar cheese.
Pop in the oven for 20-25 mins or until golden and bubbling at the edges. Alternatively, cover and freeze the pie or mini pies for another time.