Preheat the oven to 200C/ 180 fan/ gas mark 6.
Put the potatoes 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 some butter.
Season with ground black pepper.
Meanwhile, put the butter, flour and spring onions in another pan and heat gently until the butter has melted, stirring regularly.
Cook for 1 -2 mins.
Gradually whisk in the 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 minutes until thickened.
Take off the heat and stir in the cheese (if using), Norwegian cod, salmon or haddock mix, mustard, chives, sweetcorn and peas.
Spoon into an ovenproof dish or 6- 8 ramekins.
Spoon the potato on top and sprinkle with cheddar cheese.
Pop in the oven for 20 - 25 minutes or until golden and bubbling at the edges.
Alternatively, cover and freeze the pie or mini pies for another time.