Put the cumin seeds in a frying pan set over a medium heat and toast for 1-2 minutes, taking care not to burn them.
Transfer to a pestle and mortar with the salt and roughly grind, then set aside.
Peel half the skin off the cucumber in lengthways strips, creating a striped effect. Halve the cucumber lengthways, scoop out and discard the seeds and dice the flesh. Transfer to a sieve suspended over the sink, sprinkle with more salt and leave for 30 minutes.
Rinse and drain the cucumber, squeeze out more moisture with your hands and transfer to a bowl with the yoghurt. Add the spring onions and most of the walnuts, mint and grapes, and stir to combine. Season to taste with the cumin salt and black pepper.
Spoon on to small plates or into shallow bowls, then scatter a pinch or 2 of cumin salt and the remaining walnuts, mint and grapes. Serve lightly chilled with bread as a starter, or at room temperature with grilled fish.