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, melt half the butter in a saucepan. Add the leeks and a splash of water and cook for 5 minutes until soft. Stir in the flour and cook for 2 minutes. Slowly whisk in the poaching milk until smooth. Bring to the boil, whisking, then simmer for a few minutes until slightly thickened. Season well. Stir in the crème fraîche.
Divide the leek mixture between 4 individual ramekins or ovenproof dishes on a baking sheet. Arrange the haddock on top. Crack an egg into each and dot with the remaining butter. Place in a preheated oven, 180°C (350°F), Gas Mark 4, and cook for 10–12 minutes until the white is set and the yolks are still soft. Serve with toasted brown bread.