Preheat the oven to 400°F and line a baking sheet with parchment paper. Wash the whole pumpkin thoroughly and slice off the top. Cut the large pumpkin in half lengthwise from the stem end down. Use a large spoon to scoop out the stringy parts and seeds, and put this into a large bowl. Don’t worry if it won’t all come out easily; once the flesh is roasted it will be easy to scoop out the rest.
On the prepared baking sheet, place the pumpkin cut side down. Bake for about 90 minutes, or until the exterior is golden brown and the flesh is very soft when a sharp knife is inserted.
Remove from the oven and cool until the pumpkin can be handled. Scoop out any remaining pulp with a metal spoon and discard. Scoop the flesh away from the skin and discard the skin. Cool the pulp until it is cool enough to handle.
Puree the cooled roasted pumpkin in the bowl of a food processor until smooth.
Strain the puree in a fine mesh strainer set over a bowl for at least 1 hour to remove some of the moisture.
Store in quart-sized zip-lock bags or an airtight container. Refrigerate if using the pureed pumpkin within a few days, or freeze now for using later, such as Thanksgiving cakes and fall season pies.