Cook the potatoes in a large pan of boiling water for 15 minutes or so until tender.
Meanwhile, place the haddock in a pan, pour over the milk and sprinkle over the spring onions. Cover, bring to the boil and simmer for 5-8 minutes until the fish is cooked – it should flake easily. Lift out the cooked fish and discard the skin and any bones. Pour the warm milk and spring onion into a jug – you’ll need 200 ml (7 oz) and there should easily be enough.
Drain the potatoes well and mash – ideally with a ricer – until smooth. Beat in the warm milk and spring onion, followed by the fromage frais. Gently fold in the flaked fish and check the seasoning – you’re unlikely to need much, if any, salt, but grind in plenty of black pepper. Spoon the mixture into a large, shallow heatproof dish and scatter over the cheese. Pop into a hot oven for 15 minutes at 220c until golden and crunchy on the surface.