Chop the fish into chunks. Finely chop one garlic clove. Place the chopped garlic and onions in a food processor and pulse a few times. Add the fish pieces and pulse a few more times. Add the sumac, chilli, parsley and seasoning. Process again briefly – not too long, you want some texture. Cover the mixture with cling film and place in the fridge to marinate for at least half an hour.
To make the tarator sauce, place the walnuts, a pinch of salt and remaining garlic in a food processor and purée. Add the breadcrumbs, a dash of water and 2 tbsp of the lemon juice, and blend until smooth. Add a grind of pepper, then slowly add half of the olive oil, a little at a time, to make a creamy consistency. Whisk this through the yoghurt until combined.
With wet hands, roll the fish mixture into golf ball-sized balls; you should get about 20. Heat half of the sunflower oil in a frying pan over a medium heat. Shallow-fry each kofta for a few minutes on all sides until golden and just cooked through, adding more oil as needed.
Make a quick dressing from the remaining lemon juice and olive oil. Toss through the coriander, tomato, cucumber and onion with some seasoning.
Serve the hot kofta on top of the tarator sauce with the salad on the side.