Heat the butter in a pan and gently cook the leeks for 5 minutes, until softened.
Add the garlic and mushrooms and fry for 5 more minutes.
Add the stock, bring to the boil and simmer for 10 minutes.
Remove from the heat and stir in the creme fraiche. Season to taste with salt and pepper and transfer to a blender and blend until smooth. Alternatively, use a stick blender in the pan.
Dry fry the pumpkin seeds until toasted and scatter over the soup.
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