Remove the coarse outer leaves from the leek and cut off the green top and the tufty bit at the base. Cut the leek into rounds about 1cm thick and wash to get rid of any soil.
Heat the butter in a large saucepan, and add the leek and onion. Cook gently, stirring every so often, for 10 minutes, then season, add 3 tbsp water, cover and sweat for 15 minutes, stirring every so often. Keep an eye on the vegetables to make sure they don’t scorch at the bottom and add another couple of tablespoons of water if you need to.
Add the cumin and paprika and cook for another few minutes, stirring the vegetables around, then add the pumpkin and stock. Bring to the boil then turn the heat down. Season and simmer for 30 minutes, or until everything is completely soft. Let the soup cool then add the cream.
Purée the soup in batches using a blender, then pour the purée into a clean saucepan and reheat. You can judge the final seasoning better if the soup is hot.
Add the nutmeg then taste again and judge how much vinegar you should add. It’s very personal. The vinegar is supposed to heighten the flavour not make it acidic. If you’re going to serve the soup straight away, stir in the dill.
Heat the sunflower oil in a small pan and sauté the pumpkin seeds for a minute or so. Check the seasoning of the soup one last time and serve, with sour cream on top of each bowlful, a scattering of pumpkin seeds, a pinch of hot paprika and chopped dill.