To steam: Cut pumpkin in half. Remove seeds and fibers. Place cut-side down in a steamer basket, cover and cook over boiling water until a knife easily cuts through the flesh, 15 to 20 minutes. Unlike with other squash, the skin will not soften during cooking so test for doneness on the underside. Once cool, the skin will harden and separate from the flesh, making it easy to remove by hand. Transfer the flesh to a food processor and puree until smooth.
To microwave: Cut 1 to 3 pounds peeled pumpkin into 1-inch chunks. Arrange in a single layer in a microwave-safe dish large enough to hold them. Cover tightly and cook until tender, stirring halfway through cooking, about 6 minutes. Transfer to a food processor and puree until smooth.