Heat a medium saucepan over medium heat; spray with oil; add onion, cook 2 minutes or until tender. Add pumpkin and potato; cover with water; place over high heat; bring to the boil; simmer until vegetables are cooked through about 10 minutes. Drain; reserve ½ cup (125mL) cooking liquid.
Puree in a food processor or blender until smooth, adding more reserved cooking liquid if necessary to achieve the required consistency.
The recipe can be doubled and extra servings can be placed in airtight containers and frozen for later use.