Divide the fish into pieces about in square. Put the milk, water, salt and pepper, bay leaf, mace and nutmeg into a saucepan, and when warm add the fish. Bring to the boil and simmer for 10 min., then remove the bay leaf and mace. Meanwhile knead together the butter and flour, add in small portions to the contents of the saucepan and stir gently until the flour is mixed smoothly with the liquor. Simmer for 10 min., then add lemon juice, season to taste and serve.