Fry the onion and celery gently in the butter in a covered saucepan over a low heat for about 10 minutes until very tender. Tip in the rice, add the savory or thyme and cook for another minute, uncovered. Now add the broad beans, pour in 1 litre/13⁄4 pints stock and season with salt and pepper. Bring to the boil, simmer for about 5 minutes, then tip in the peas and cook for a further 5 minutes, by which time the rice will be tender.
Remove the herb sprig and purée the soup in batches in a blender or food processor, then return to the rinsed-out pan. (If you’ve got a hand blender, use it in the pan.)
Shortly before serving, reheat the soup thoroughly and add more stock if the consistency is too thick for your liking. Take the pan off the heat and stir in 1 tbsp of the yogurt and the chopped mint. Continue to stir in the yogurt, 1 tbsp at a time, until it’s all incorporated. Taste for seasoning, then serve in warmed bowls with an extra spoonful of yogurt stirred into each and a grinding of black pepper on top.