1. Roughly chop the onion. Wash, cut, hollow out and roughly dice the Hokkaido. Peel and roughly dice the celery. Wash the lemongrass and cut it in half lengthwise.
2. Heat olive oil in a pot. Add the onion, Hokkaido and celery to the pot and sauté.
3. Add water and lemongrass to the pot and simmer for half an hour.
4. Remove the lemongrass from the soup again.
5. Dissolve the locust bean gum in 200ml cold water.
6. Add the dissolved locust bean gum and bring the soup to the boil briefly, stirring constantly.
7. Puree everything well.

Tip : For the best results, you can use a hand blender to dissolve the locust bean gum in the cold water.