Wash, peel and cut the potatoes in cubes. Put them in a pan of cold water. Bring to the boil and cook for 20-25 minutes.
Drain the potatoes thoroughly and mash them with the splash of milk and knob of butter. Season with ground pepper.
Put the 25 g of butter, 25 g plain flour and 4 finely sliced scallions into a different pan and heat until the butter has melted (1-2 minutes).
Whisk in 400 ml of milk and bring to the boil. Avoid lumps by stirring. Cook 3-4 minutes until thickened.
Remove from the heat and stir in the 320-400 g of seafood mix of fish. Then add the teaspoon of Dijon mustard, the small bunch of chives, the sweetcorn, and the peas.
Spoon the mixture into an ovenproof dish and spoon the potato on top. Sprinkle the mature Cheddar cheese over the potato.
Put into the oven and cook for 20-25 minutes until golden brown and bubbling.
