Simmer milk, onion, nutmeg, bay leaf, and peppercorns in a pan, then cool for 10 minutes.
Melt 25g butter in a pan, whisk in flour to make a roux, and cook for one minute.
Strain the cooled milk mixture into the roux and whisk until smooth.
Boil the mixture while whisking until it thickens enough to coat a spoon.
Stir in 10g butter, simmer for 3 minutes, add parsley, and season.
Crush tender potatoes and celeriac with butter and cream until smooth, then season.
Season flour on a plate and dredge the cod fillets until fully coated.
Fry cod in hot oil for 3 minutes per side until golden and opaque.
Add butter and lemon juice to the cod pan, then remove from heat once melted.
Plate the mash, top with cod, drizzle pan juices and sauce, and garnish with lemon.