Melt the butter in a heavy-based pan and add the onion and a good pinch of salt. Sweat for 15 minutes until soft, then stir in the rice and add enough water to cover the rice by 3cm.
Bring to the boil, then turn down to a simmer and leave for 10 minutes until al dente.
Once the rice is cooked, stir in the chickpeas, the chopped herbs, spring onions and garlic.
Whisk the eggs yolks into the yogurt, then slowly stir into the soup.
Add 300m water, season well, then gently bring to a simmer, making sure it never boils.
Meanwhile, in a small pan, melt the butter and gently fry the garlic until it just begins to colour, then stir in the mint and give it a moment to crisp and curl up. Take off the heat.
Transfer the soup to bowls, then drizzle each bowl with a spoonful of the garlicky mint butter.