Heat the oven to 200C/ fan 180c/ gas 6.
Put potatoes, peeled and halved, in a sausepan 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 the 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. Bring to the boil, stirring to avoid any lumps and sticking. Cook for 3-4 minutes until thickened..
Take off the heat and stir in your mixed fish, 1 tsp English mustard, a small bunch of finely chopped chives and a handful of sweetcorn. Spoon into an ovenproof dish.
Spoon potato on top and sprinkle with a handful of cheddar cheese.
Pop in the oven for 20-25 minutes or until golden and bubbling at the edges..
Serve with green vegetables of choice.