Trim the roots and top 3cm off the leeks. Halve them, rinse out any grit and finely slice them. Trim and thinly slice the celery stick. Fill and boil your kettle.
Warm 1 tbsp olive oil in a large pan on a medium heat. Add the leeks and celery with a pinch of salt and pepper. Lower the heat and fry for a few mins, stirring often.
Peel, core and finely dice the apple. Fold the apple and 2 tsp fennel seeds into the pan of veg. Fry, stirring often, for 4-5 mins, till the apple starts to break down and caramelise.
While the apple fries, drain and rinse the butter beans. Pick the thyme leaves off their sprigs, setting a couple of pinches aside for garnish.
Add the butter beans and thyme to the pan. Crumble in the stock cube and pour in 750ml hot water from the kettle. Simmer for 10-15 mins till the veg are tender.
To blitz the soup smooth, you can ladle it into a blender, or use a hand-held blender in the pan, adding a splash more water if it’s a little too thick. Alternatively, you can leave the soup chunky. Taste and add more salt or pepper if needed.
Ladle the soup into a couple of warm bowls, and serve topped with the reserved thyme leaves.