Heat the oil in a pot then finely dice the onion and garlic and fry until the onion starts to soften.
Turn the heat to medium, then add the butter to the pot and heat until melted.
Stir in the flour and cook for about 1 minute before slowly whisking in the milk to from a smooth white sauce.
Cut the frozen fish into chunks and add to the sauce, along with the garlic powder, peas and sweetcorn. Give the sauce a good stir, then leave aside.
To make the mash, peel and cut the potatoes into chunks, cover with water, then boil for 10-15 minutes until tender. Drain then mash with the butter and milk, then finely chop the spring onions and season with little black pepper.
Pour the fish mixture into an ovenproof dish, then spoon over the mashed potato. Using the back of a fork fluff up the potato so that it crisps up with baking.
Bake the pie for 30 minutes until the top of the potato is golden brown.
Serve with finely chopped parsley and a little extra black pepper sprinkled over.