Preheat the oven to 200 degrees (180 fan forced). Line a flat baking tray with baking paper.
Cut the pumpkin into large pieces (and remove the seeds and skin) and place onto the baking tray with the quartered onion.
Drizzle with the olive oil and season with salt and pepper.
Cook for 35-40 minutes or until the pumpkin is soft.
Place the pumpkin and onion into a saucepan and allow to cool slightly.
Add 1 cup of vegetable stock and blend with a stick blender (a normal blender or a Thermomix can be used in place of the stick blender).
Add the remaining 2 cups, one at a time, blending in between each addition.
Add the cream and gently stir through.
Bring the soup in a saucepan to a low simmer until heated through.
Serve with a dollop of sour cream.