Thinly slice the white and light green parts of the leeks. Rinse well. Thinly slice the celery.
Warm 1 tbsp oil in a pan over a medium heat. Add the sliced veg and season well. Lower the heat. Fry for a few mins, stirring often.
Peel, core and finely dice the apple. Fold the apple and 2 tsp fennel seeds in to the veg. Sizzle till the apple starts to break down and caramelise.
While the apple fries, drain and rinse the beans. Finely chop 3 sage leaves. Pluck out a few flat, pretty sage leaves and set aside for later.
Add the butter beans, chopped sage, the stock cube and 750ml boiling water to the pan. Simmer till the pan starts steaming and the veg are tender. Blend the soup till smooth, either using a blender or a hand-held blender in the pan. Add a little more water, if needed, till it’s the right consistency for you. Season to taste.
Set a small frying pan over a high heat. Add 1 tbsp oil to the pan. Fry the reserved sage leaves with a pinch of salt for 1-2 mins till golden and crisp. Scatter over the soup as a garnish.