Heat remaining 2 tbs oil in a large saucepan over medium-high heat. Add onion and cook, stirring, for 4-5 minutes or until onion is softened. Add pumpkin mixture, then cover with stock. Bring to a boil, reduce heat to medium and simmer, stirring occasionally, for 15-20 minutes. Remove from heat and, using a stick blender, whiz until smooth. Season and stir in lemon zest and juice.