Finely grate the cucumber with the skin and cut the tomatoes into small cubes. Put both in a sieve and let the excess liquid drain off. Then mix with the yoghurt in a bowl and season with salt and a little pepper.
Heat the oil in a small pan over medium heat. Add the mustard seeds, cover and shake until they pop, then pour over the yogurt. Serve sprinkled with coriander leaves if you like.