Fill a deep-fat fryer or a large, deep, heavy-bottomed pan two-thirds full with vegetable oil. Heat the oil to 150/C/300F (use a probe thermometer to check the temperature if you're not using a deep-fat fryer). (CAUTION: Hot oil can be dangerous. Don't leave unattended)
Place125ml/4½fl oz water and the butter into a saucepan and bring to a boil for one minute.
Remove from the heat and add the flour in one go – beating well until the mixture is smooth and comes away from the sides of the pan. Return to the heat for a minute or two and cook through.
Remove from the heat again and add the eggs, one at a time, beating well between each egg.
Add the potato, feta, basil and parsley and mix well then season with salt and black pepper. Chill the mixture in the fridge for 30 minutes, or until just firm enough to mould.
Place a small ball of choux over the head end of the sardine, covering to about half way up the fish, leaving the tail end exposed.
Fry the fish in batches for 4-5 minutes, or until golden-brown and cooked through. Carefully remove the fish from the fryer with a slotted spoon and drain on kitchen paper.
For the salad, place the tomatoes onto four separate serving plates in a circle.
Scatter over the onion, olive oil, basil and balsamic. Season with salt and freshly ground black pepper. Add the rocket and place the sardines on top of the salad. Finish with a squeeze of lemon and serve immediately.