In large pot, heat oil over medium heat. Add onion, salt and pepper and cook, without browning, until tender and translucent, about 10 minutes. Stir in curry paste and cook one minute more.
Stir in broth and bring to boil over medium high heat. Add pumpkin and simmer, over medium heat until pumpkin is very tender, about 20 minutes. Transfer pumpkin with liquid to blender, add walnuts and process in batches until smooth. The mixture will remain quite thick.