Hot Yoghurt and Broad Bean Soup

Description
This recipe is from Yotam Ottolenghi's vegetarian cookbook "Plenty". It was published in guardian.co.uk
 * Serves four.

Ingredients

 * 6 tbsp good-quality olive oil
 * 1 medium onion, quartered
 * 4 celery sticks, quartered
 * 1 carrot, cut into 2 cm chunks
 * 5 thyme sprigs
 * 2 bay leaves
 * 30 g flat-leaf parsley
 * 500 g (podded weight) broad beans
 * 50 g long-grain rice
 * salt and white pepper
 * 400 g Greek yoghurt
 * 2 garlic cloves, crushed
 * 1 large free-range egg
 * 3 tbsp each chopped dill and chervil
 * grated zest and juice of 1 lemon

Directions

 * 1) Start by making a stock.
 * 2) Heat two tablespoons of oil in a large pot and over medium heat sauté the onion, celery and carrot for five minutes – you want to soften the vegetables without browning them.
 * 3) Add the thyme, bay and parsley, cover with 1.2 litres of water and bring to a boil.
 * 4) Lower the heat, cover and simmer gently for 30 minutes.
 * 5) Meanwhile, prepare the beans.
 * 6) Bring a saucepan of water to a boil, add the beans, simmer for a minute only, drain and refresh under running cold water.
 * 7) Next, remove the skins by gently pressing with your fingers against the sides of each bean, causing the soft bean inside to pop out.
 * 8) Pass the cooked stock through a sieve into a medium pan, add the rice and bring to a boil.
 * 9) Simmer, covered, for 20 minutes, then add half the skinned broad beans, season and blitz with a stick blender until smooth.
 * 10) In a large, heatproof bowl, whisk together the yoghurt, garlic and egg.
 * 11) Add a ladleful of hot soup and whisk.
 * 12) Continue gradually adding soup until you have mixed in at least half of it (do this slowly, otherwise the yoghurt might split due to the difference in temperature).
 * 13) Pour the tempered yoghurt into the soup pan, place over a medium heat and warm up while stirring constantly.
 * 14) Make sure the soup doesn't boil.
 * 15) Taste, add more salt and pepper, if you like, and ladle into four shallow bowls.
 * 16) Drop in the remaining beans, garnish generously with dill, chervil and lemon zest, and drizzle with the remaining four tablespoons of olive oil (this is important), plus some lemon juice, if you like.