Heat 2 tablespoons of water in the base of a large saucepan. Add the leeks, celery and celeriac along with a pinch of salt and sauté over a medium heat for 5 minutes, stirring from time to time so they don't stick.
Add the garlic and a little more water if needed and cook for 2 minutes.
Add the herbs and pour in enough stock to cover all the veggies. Bring to the boil, then reduce the heat and simmer for 20 minutes with the lid on until the veg are soft.
Add the drained beans and celery salt to pan, along with more stock so everything is covered. Simmer for another 3 minutes.
Turn off the heat and leave to cool for a few minutes. Blend to smooth using a stick blender, adding the rest of the stock if the soup is too thick.
Season with salt and pepper to taste.
Serve with a glug of flaxseed oil and/or an extra pinch of celery salt and/or a sprinkle of chopped chestnuts.