Step 1

Mix the salt with the rosemary, then scatter half in a ceramic baking dish. Top with the cod, skin-side down, then cover with the remaining rosemary salt. Cover with cling film and chill for 12-24 hours, turning halfway through.

Step 2

Remove the fish from the salt, put in a bowl, cover with cold water, then soak for 30 minutes. Drain and repeat with clean water, then rinse and dry the fish with kitchen paper.

Step 3

In a large bowl whisk the egg and milk until smooth. Scoop out the fluffy insides from the potatoes, add to the bowl and whisk again until smooth (see tips). Add the flour and parsley and mix until thick, flake in the cod (discarding the skin) and gently fold to combine for a smooth batter. Add a dash more milk if needed. Mix the sauce ingredients together, season and set aside.

Step 4

Pour oil into a high-sided saucepan until a third full, then heat to 180°C on a digital probe thermometer or until a cube of bread turns golden in 30-40 seconds. Pat the capers dry, then fry for 1 minute until they open like flowers and are just crisp. Drain on kitchen paper.

Step 5

Heat the oil to 180°C again. Gently spoon heaped tablespoons of batter into the hot oil and fry, in batches, for 2 minutes until golden and crisp. Remove with a slotted spoon, drain on kitchen paper, then serve with aïoli and crispy capers.