Heat the oil in a large lidded pan. Add the onion, garlic and leeks and cook for 5 minutes until just softened. Add the potatoes and pour in the stock. Bring to the boil, reduce the heat and cover. Simmer for 15-20 minutes until the potatoes are cooked through.
Remove the soup from the heat and stir in the thyme. Using a blender or stick blender, thoroughly process until smooth. Return the soup to the pan and stir in the chopped kale. Bring back to the boil, then simmer for a further 5 minutes. Season to taste.
Meanwhile, heat a nonstick frying pan over a high heat. Add the slices of prosciutto and cook for a minute on each side to crisp up. Ladle the soup into mugs or bowls and drizzle in a little soured cream (if you like), then serve topped with crumbled prosciutto and some fresh crusty bread on the side.
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