Heat the oven to 400°F (200°C). Wash the pumpkin, cut it in half from stem to base, and scrape out the seeds and stringy fibers; reserve the seeds for roasting if desired.

Place the pumpkin halves cut-side down on a rimmed baking sheet or in a shallow roasting pan. Add about ½ inch of water to the pan and roast for 45–60 minutes, until the flesh is very tender and easily pierced with a fork.

Cool the roasted pumpkin until it is comfortable to handle. Scoop the flesh away from the skin and process it in a food processor or blender until smooth; a food mill or potato ricer also works.

If the purée seems watery, drain it in a fine-mesh sieve lined with cheesecloth for 30 minutes to several hours. Use immediately, refrigerate in an airtight container for up to 4 days, or freeze in measured portions for up to about 3 months.