Preheat the oven to 400°F / 200°C / Gas Mark 6, no fan (if you have a convection oven reduce the temperature by about 20° or follow the manufacturer's instructions).
Cut the pumpkin into a couple equal-sized pieces and place on a baking sheet. There is no need to peel the pumpkin.
Scoop out the seeds using a spoon.
Bake for about 40-60 minutes. The baking time will depend on the thickness of the pumpkin pieces. To check if the pumpkin is ready, stick a fork in its thickest part - you should be able to stick it without resistance.
Scoop out the flesh out of the skin.
Puree the flesh until smooth. You can do that in a blender, food processor, or using a hand (immersion) blender.
The pumpkin puree is now ready to be used.