Trim the roots and top 3cm off the leeks. Halve them, rinse out any grit and finely slice them. Roughly tear the mushrooms. Place a large pan on a medium heat and pour in 2 tbsp olive oil. Add the leeks and mushrooms to the pan with a pinch of salt and fry for 8 mins, stirring often, till the veg are softened and juicy.
While the veg cook, peel and grate and crush the garlic. Finely chop the sage leaves. Peel the celeriac, slice off the roots and chop the celeriac into bite-size chunks. Trim and peel the carrots and chop them to match. Fill and boil the kettle.
Add the garlic and sage to the pan. Cook for 2 mins, stirring every now and then. Drain and rinse the butter beans.
Add the celeriac, carrots and butter beans to the pan. Pour in 800ml hot water from the kettle and crumble in the stock cube. Stir to mix and pop on a lid. Bring to the boil, then turn the heat down and simmer for 20 mins till the veg is tender when pierced with a knife.
While the hot pot simmers, strip the kale leaves off their cores and roughly chop the leaves. Finely grate the lemon zest. Cut the lemon into 6 wedges.
When the hot pot has cooked for 20 mins, ladle about a quarter of it into a blender and blitz till smooth, then stir it back into the pan. If you have a stick blender, you can use it directly in the pan to blitz a bit of the hot pot smooth. You could also use a masher in the pan to crush up some of the chunky veg. This helps to thicken the hot pot up a little, but you can skip this step if you prefer.
Stir the kale and lemon zest into the hot pot. Simmer for 5 mins to wilt the kale. Taste and add more salt or pepper if you think it needs it. Serve in warm bowls with plenty of black pepper and the wedges of lemon to squeeze over.