For the leek and potato soup, heat the butter in a pan and sauté the onion, garlic and leek until softened.
Add the potato and stock and bring to the boil. Reduce the heat and simmer for ten minutes, or until the potato is tender. Remove from the heat and allow to cool slightly.
Pour the soup into a blender and blend until smooth.
For the spicy pork fat, heat the pork fat in a pan with the garlic and paprika. Season with salt and freshly ground black pepper and fry gently until the fat has rendered and the texture is crisp. Remove the fat pieces with a slotted spoon and drain on kitchen paper.
To serve, pour the soup into a bowl, top with the spicy pork fat and garnish with chervil.