Place the haddock and milk in a saucepan and poach the fish for 3–4 minutes or until cooked. Remove with a slotted spoon, then remove the skin and any bones. Flake the fish into a bowl.
Beat the eggs in a separate large bowl and season.
Heat a quarter of the butter in a frying pan until foaming, then pour in a quarter of the beaten egg. Stir a little with a fork, tipping the pan so the egg covers the base, then cook for 3–4 minutes until set.
Place a quarter of the spinach on the omelette, then top with a quarter of the fish. Fold the omelette over and cook for a further minute. Serve on a warm plate.