Preheat oven to 400 degrees. Snap off the pumpkin's stem and halve the gourd lengthwise. With a spoon or melon baller, remove seeds and rinse for roasting or discard.
Place pumpkin halves cut-side down on a large rimmed baking sheet. Roast until tender, about 1 hour.
When cool enough to handle, scoop out cooked pumpkin flesh; discard skin. Transfer pumpkin flesh to a food processor; process until smooth.
Set a colander in a large bowl and line with a double-layer of cheesecloth. Place pumpkin puree in cheesecloth. Cover with plastic wrap and place in refrigerator to drain, at least 4 hours, and up to 3 days.
Kept in the freezer, the puree will last for up to 6 months.