Thinly slice the radishes and put them in a sieve. Shave the courgette into thin, lon strip using a vegetable peeler. Put these in the sieve too. Drizzle with the lemon juice, season with salt and mix well. Let it stand over a bowl for at least 10 minutes, or up to 30 minutes if you have time. Drain, and put them in a serving bowl.
Finely chop the spring onions, add to the bowl.
Mix the yoghurt with the dill and garlic.
Pour this over the salad, toss well and season with freshly ground pepper. Taste, and add more lemon juice of needed.
Serve with olive oil drizzled over the salad.