Simmer the rice in the vegetable stock for 15 minutes, then add the carrots and zucchini and cook for another 5-6 minutes. If there is still stock in the pot, drain it off. Let the rice and vegetable mixture cool down.
Then add the radishes, spring onions and parsley and mix everything.
Make a dressing from yogurt, oil, vinegar, mustard and spices and add to the salad. Mix well and let steep in the refrigerator for a while.
You can also prepare the salad the day before. Season again to taste before serving.