Trim the roots and the top 3cm from the leek. Slice a cross into the top and open the leek out to rinse out any grit. Finely slice it. Trim the very ends off the celery and finely slice it. Peel the carrot and finely chop it. Peel the onion and finely chop it.
Warm a large pan for 1 min, then add ½ tbsp olive 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 800ml 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. Once boiling, turn the heat back down a little and simmer for 30 mins.
Slice the thick cores out of the kale. Finely shred the leaves and add them to the pan. Simmer for 5 mins without the lid (this keeps the kale green). The pearl barley should be tender. Taste and add more salt or pepper if it needs it. Ladle into warm bowls and serve.