In a small bowl, combine the thinly sliced red onion with sumac, vinegar and a pinch of salt. Leave to sit for at least 30 minutes.
Trim the ends off the cucumbers, then using mallet or the flat of a large knife, bash the whole cucumbers a few times until they split, bruise and break. Scrape the seeds out, then chop or break the cucumber into random, bite-sized pieces.
Transfer to a bowl with half a teaspoon of salt and leave to sit for 20 minutes, to draw out the moisture from the cucumber. Drain, then add the lemon zest and juice, half the coriander seeds, garlic, radishes, ½ the herbs, sumac'ed onions, olive oil, a quarter-teaspoon of salt and black pepper.
Whisk and spread a thin layer of Greek yoghurt all over a serving platter. Place the salad on top, sprinkle with the remaining coriander seeds and garnish with the remaining chopped herbs.