Simmer 500ml milk with sliced onion and bay leaf in a large pan.
Poach haddock skin-side down for two minutes, then flip and simmer for two more minutes.
Boil a saucepan of water with vinegar and salt, then create a vortex.
Crack one egg into the vortex, cook for three minutes, then chill in iced water and repeat.
Remove haddock from milk and keep warm in a low oven.
Strain the milk into a jug and wipe the pan clean.
Melt 25g butter, stir in flour for 3 minutes, then gradually whisk in the reserved milk.
Whisk until smooth, then stir in mustard, lemon juice, and seasoning.
Wilt spinach in a hot pan with the remaining butter and drain on paper.
Reheat the poached eggs in simmering water for 30 seconds.
Assemble by layering spinach, haddock, and eggs, then top with mustard sauce.