Peel and finely chop the onion. Trim the root and top 3cm of the leek. Slice in halve vertically, and rinse any dirt out under a cold tap. Finely slice. Scrub the carrot and cut into dice. Peel the garlic cloves and crush or finely grate.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the onion and leek, and sprinkle over a pinch of salt and pepper. Cook for 4-5 mins, stirring occasionally, till a little softened. Stir in the garlic and cook for a further minute.
Pick the thyme leaves and add to the pan along with the chopped carrot. Pour in the pearl barley and cover with 850ml boiling water. Crumble in the stock cube and stir everything well to combine. Return to the boil, then reduce to a simmer and cook gently for 20 mins, till the barley is tender.
Pull the kale leaves from the thick stalks. Discard the stalks, and shred the leaves. When the barley is cooked, stir the kale into the broth for a further 3-4 mins, till wilted.
Have a taste and add more salt and pepper if needed. Ladle the scotch broth into deep bowls and serve.