Using a large chopping knife of cleaver slice the pumpkin in half from the bottom up to the stem. The stem is very tough so instead of trying to cut through it grab the bottom of the pumpkin and pull each side apart. The pumpkin will break open at the base of the stem.
Preheat oven to 400F. Use the edge of a large spoon to scrape out the pulp. Discard the pulp or set it aside to make roasted pumpkin seeds later.
Place the cleaned pumpkin halves on a baking sheet. If they do not lay flat you can shave a slice off the back part so they will sit upright on the flattened pumpkin surface. Roast for 45-50 minutes or until the pumpkin flesh is soft. Remove from oven and let cool to the touch and then use the same spoon to scoop out the pumpkin flesh. Discard the shells.
Add the cooled pumpkin flesh to a blender and pulse to blend. Depending on the size of the pumpkin and power of your blender you may need to work in batches. Blend until smooth. Transfer to a resealable dish.
Homemade pumpkin puree is thinner than canned. If a thicker consistency is required place the pumpkin puree into a fine sieve over a bowl and slowly drain away any excess liquid. After a few hours discard the liquid and then transfer the thickened pumpkin puree back into a resealable container.
Storage: Store in the fridge in a sealed container for up to 7 days or freeze for up to 5 months.