Put the potatoes into a pan of boiling water and cook for 15-20 minutes. Test them with a skewer and it should go in easily.
Remove from the water and drain in a colander. Mash with butter, milk, salt and pepper.
Preheat the oven to 180C, gas mark 4.
For the fish pie mix, heat the butter in a pan and cook the onion until soft.
Add the fish pie mix, cook for 1 minute, then add the mustard and boil for 1 minute.
Heat the milk in a pan, careful not to boil. Mix the flour into the milk mixture. Remove from the heat and add the butter.
Also add the finely snipped chives, a handful of sweetcorn and a handful of petits pois.
Pour the fish mixture into an ovenproof dish or 6 ramekins.
Spoon the mashed potato on top of the fish mixture and then sprinkle the grated cheddar on it.
Place into the oven at 180c for 25 minutes or until golden brown and edges are bubbling.
Serving Suggestions: Serve with a Savoy Cabbage, which works well as a side dish. This recipe also goes well with quick braised lettuce and peas and carrots.