Heat oven to 220C/200C fan/ gas 7 and line a baking tray with parchment. Cut the whole potato into matchsticks using a julienne peeler, or shave thin slices using a vegetable peeler, then cut into matchsticks. Pat dry using kitchen paper, then toss with the oil and some seasoning. Spread out on the tray and roast for 15-18 mins.

Melt the butter in a large saucepan. Add the leeks, chopped potatoes and a pinch of salt, then cook gently for 10 mins until the leeks have softened. Stir in the garlic and cook for 1 min more, then pour in the stock. Simmer for 10-12 mins until the potatoes are soft, then add the kale and cook for 2-3 mins to wilt.

Stir in the crème fraîche, then blitz with a hand blender and season to taste. Divide the soup between bowls and top with the shoestring fries.