Heat vegetable oil in a deep fryer or heavy pan to 150°C.
Boil 125ml of water and butter in a saucepan for one minute.
Beat in the flour until smooth and cook on the heat for two minutes.
Add eggs one at a time, beating well after each addition.
Mix in potato, feta, and herbs, season, then chill for 30 minutes until firm.
Mould choux mixture over the head half of each sardine, leaving the tail exposed.
Fry fish in batches for 5 minutes until golden and drain on paper.
Arrange sliced tomatoes in circles on four plates.
Dress with onion, oil, vinegar, herbs, season, and top with rocket and sardines.