In a large pan, over a low heat, gently heat the oil and add the white part of the leeks, onions, garlic.
Season with salt and pepper, then cook for about 3 - 5 minutes until softened without colouration. Add the potatoes and the vegetable stock and bring to boil. Cover with a lid and let the soup simmer very gently for a further 15-20 minutes or until the vegetables are softened.
In the meantime, in a second saucepan filled with boiling water. Add the bicarbonate of soda and salt. Add the green part of the leek and boil for 3-5 minutes until just softened. The bicarbonate of soda will help keep the green colour of the leeks. Remove from the heat into a colander and cool the leeks to stop them overcooking in cold water for 2 minutes. Remove from the water and keep aside until needed.
When the vegetables in the soup saucepan are soft, remove from the heat and add the boiled green leek pieces.
Using a stick blender, process the soup until smooth. Check the seasoning and add the cream. Check the texture and add some water if needed to loosen the texture.
Serve the soup with a swirl of extra cream, sprinkle with fresh herbs and some fresh bread.