Cut pumpkins lengthwise into quarters, removing seeds and stringy pulp. Arrange pumpkins, cut sides down, in each of 2 (2-quart) casseroles. Add 1/2 cup water to each casserole; cover and microwave at high 15 minutes. Cool; peel pumpkins. Place pumpkin pulp in a food processor; process until smooth or mash with a potato masher.
Note: Store remaining purée in a zip-top bag in refrigerator up to a week or in freezer up to 6 months.