Put the frozen mashed potato in a microwave-safe bowl with 2 tbsp water. Cover with cling film and cook on high for 8-9 minutes, stirring every 2-3 minutes until thoroughly defrosted and piping hot. Set aside.
Meanwhile, put the frozen fish mix into a large deep frying pan and cover with cold water. Bring to the boil, then simmer for 2-3 minutes until the fish flakes easily. Remove the fish with a slotted spoon and transfer to a 2-litre ovenproof baking dish. Break into small chunks with a fork. Reserve 150ml of the poaching liquid.
Heat the olive oil in a large frying pan, add the frozen onion, garlic, ginger, and chilli and fry over a medium-high heat for 8-10 minutes, stirring occasionally, until the onions are golden and any excess moisture has evaporated. Stir in the balti paste and cook for a further 1 minute.
Reduce the heat and gradually stir in the crème fraîche and reserved poaching liquid. Stir in the peas and chopped coriander and simmer for 1-2 minutes. Pour the sauce over the fish and gently mix together.
Spoon the mashed potato over the filling to cover completely – using the prongs of a fork to spread it evenly. Place on a baking tray and bake in theoven for about 35 minutes until golden and bubbling.