Wash the pumpkin thoroughly and slice off the top. Cut the pumpkin in half lengthwise from the stem end down. Scoop out the strings and seeds. 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 a lightly oiled or parchment-lined heavy baking sheet, place the pumpkin cut side down. Turn on the oven to 400 degrees, and place the pumpkin in. Pre-heating doesn’t matter when roasting like this because the pumpkin can begin to roast from the lower temps on up to temperature
Roast for about 90 minutes, or until the flesh is very soft and a knife inserted meets no resistance. Remove from the oven and cool until the pumpkin can be handled. Scoop out any remaining pulp and discard. Scoop the flesh away from the skin and discard the skin.
Puree the cooled roasted pumpkin in a food processor until smooth. Store in quart-sized zip-lock bags. Refrigerate if using the puree within a few days, or freeze now for Thanksgiving cakes and pies. If the puree is very watery, drain briefly in a cheesecloth or sauté in a pan on low heat to remove some of the moisture.