Trim the roots and thick green tops from the leek. Slice a cross into the top and open the leek out to rinse out any grit. Finely slice the leek. Trim the very ends off the celery and finely slice it. Peel the carrot and finely chop it. Warm a pan for 1 min, then add ½ tbsp oil and the chopped veg. Season with a good pinch of salt and pepper. Cover. Keep the heat very low and gently sweat the veg for 5 mins till soft. Stir now and then.
Peel and grate or crush the garlic. Crumble the stock cube into a heatproof jug and stir in 850ml boiling water to dissolve it. Stir the garlic into the veg with the bay leaf and pearl barley. Pour in the stock. Cover, turn up the heat and bring to the boil. Turn the heat back down a little and simmer for 30 mins.
Slice the thick cores out of the kale. Finely shred the leaves. Drain and rinse the butter beans. Add them to the pan. Simmer for 5 mins without the lid (to keep the kale green). The pearl barley should be tender. Taste and add more salt and pepper if it needs it. Ladle into warm bowls to serve.