Begin by washing the leek thoroughly then roughly chop into even sized pieces. Slice the spring onion. Heat the butter and sunflower oil in a large saucepan on a medium-low heat. Add the leeks and spring onions and a pinch of salt. Stir and sauté without colour for several minutes. Set two asparagus spears to one side for garnishing, slice the rest. Add the sliced asparagus to the saucepan and cook for 3-4 minutes on a medium-low heat. Add the hot stock to the vegetables, salt and white pepper, bring to the boil, simmer until the asparagus are just cooked, this shouldn’t take longer than 3-4 minutes. Remove from the heat and blitz in a food processor immediately. Pour into a bowl set over iced water to chill quickly and retain the lovely bright green colour.
Once the soup has cooled, add the Greek-style yogurt and whisk with a balloon whisk to incorporate the yogurt. Check the seasoning, add more salt and white pepper if necessary.
Bring a pan of water to the boil and add a generous sprinkle of salt, drop the asparagus into the water and simmer for 2-3 minutes. Remove and cool in iced water. Slice finely lengthways.
Pour the soup into four bowls, drizzle a little extra yogurt in the soup. Add the sliced asparagus, chive flowers and optional extras.