Note: you’ll need to sterilize jars beforehand. See here for guidance.
Preheat the oven to 350°F (180°C). Line your baking tray with aluminum foil.
Slice the pumpkin, scoop out the seeds, cut into chunks. Then place them cut-side down on the tray.
Bake between 40 minutes to an hour. The time will depend on the size - check with a fork and see if it’s nice and soft.
Set aside to cool, then scoop out all the flesh. Blend until smooth with a food processor. Move purée to a deep cooking pan, squeeze in lemon juice, add vanilla and cinnamon. Stir well, then add sugar. Simmer on low heat for 10 minutes.
Remove from stove. Carefully divide the jam between the jars with a spoon. Seal the jars tightly. Boil in water bath for half an hour. After that, set them aside to cool down.
If all went well, the pumpkin jam should be good to eat for up to 12 months. When opened, keep in the fridge and eat within 2 weeks.