Preheat oven to 350 degrees. Split pumpkins in half vertically. Scoop out seeds and membranes. Save seeds for later use, if desired. Place pumpkin halves cut-side down on parchment-lined baking sheet.
Poke the flesh of the pumpkins several times with a fork. Place in preheated oven and roast for about 45 minutes, until a fork is easily inserted into pumpkin and comes out easily.
Turn pumpkins over, scoop out flesh all the way to the skin, and put flesh into food processor. Process until fibers are broken down and mixture is smooth. Put purée in fine-mesh strainer over a bowl or in a cheese-cloth-lined colander. Allow to drain about two hours.