Set pumpkin on a large, secured cutting board. Use a long knife to carefully cut it in half. Scrape off the stringy material and seeds attached to the walls of the pumpkin with a large metal spoon. Cut each half piece of pumpkin in half again. Save the seeds for roasting, if desired.
Preheat the oven to 300°F. Place the pumpkin pieces, skin-side down, in a large roasting pan (or two if very large). Pour in one inch of water, tightly cover and bake 2 to 2¾ hours, or until very tender.
When cool enough to handle, remove the pumpkin peel and purée the flesh in a food processor. If your purée is watery, spoon it into a wide, non-stick skillet and stir it over moderate heat until the excess water evaporates and the purée is thick as canned pumpkin.
Cool the purée to room temperature before using in such things as soups, sauces and baked goods.
You could also freeze the pumpkin for another time.