Pour the milk over the haddock in a shallow saucepan. Simmer for 7–10 minutes until the fish flakes easily. Strain the milk through a sieve into a jug. When the fish is cool enough to handle, tear into flakes, discarding the skin and any bones.
Meanwhile, grease 4 individual soufflé dishes well with some of the butter and dust with the Parmesan. Place on a baking sheet. Melt the remaining butter in a saucepan. Add the flour and cook for 2 minutes. Slowly whisk in the poaching milk until smooth. Bring to the boil, whisking, and when it starts to bubble and thicken, take off the heat and stir in the Gruyère and fish flakes. Stir in the egg yolks, one at a time, then leave to cool a little.
Whisk the egg whites in a grease-free bowl until stiff and glossy. Stir one-third into the fish mixture, then carefully fold in the remaining mixture in 2 batches along with the spring onions. Spoon into the soufflé dishes. Place in a preheated oven, 200°C (400°F), Gas Mark 6, for 10–12 minutes until well risen. Serve immediately.