Halve haddock fillets and place in a pan with milk, bay leaves, and peppercorns.
Bring milk to a boil, remove from heat, cover with a lid, and let infuse.
Fry chopped onion in butter over low heat.
Add carrot, swede, potato, mustard seeds, and turmeric to the pan and fry until lightly browned.
Remove fish from the milk and set aside, reserving the liquid.
Dust vegetables with flour and cook for two minutes.
Pour reserved milk over vegetables and stir constantly until sauce thickens.
Simmer until vegetables are softened.
Return haddock to the pan to heat through.
Stir in chopped parsley and serve.