Skin the filleted fish. Make a fish stock with the water, bones, head and skin of the fish, salt, lemon rind, herbs and mace, simmer gently for ½ hr. Cut the fillets into 2-in. strips, slice the onions thinly and dice the potatoes. Cut the pork in tiny cubes and heat it gently in a deep pan until the fat flows freely. In the pork fat cook the onion without browning for 10 min., then add the potatoes and shake them well in the fat for a few minutes. Sprinkle in the flour. Gradually add the hot, strained stock, then put in the pieces of fish and season the soup. Cook very gently for ½ hr. Heat the milk and butter and add them to the soup when the fish and potatoes are soft. Do not reboil. Serve at once.