Melt the vegan butter in a large pot over medium heat. Add the onion and cook for 4–5 minutes, until softened and translucent.
Add the garlic and cook for about 30 seconds, just until fragrant.
Add the pumpkin and vegetable stock. Bring to a boil, then reduce the heat and simmer for about 20–25 minutes, or until the pumpkin is completely tender.
Blend the soup until completely smooth and creamy using an immersion blender. If using a regular blender, work in small batches and vent the lid so steam can escape.
Stir in the vegan sour cream, salt, black pepper and nutmeg. Blend or stir again until fully combined.
Taste and adjust the seasoning if needed. Serve hot, with an extra spoonful of vegan sour cream if you like.