"Although Maniere called this a "cheese cake," it's nothing like our rich, dense American version. His lemony pumpkin-flavored dessert is low-fat, airy, and souffle-like because it's made with only the beaten whites of the egg. The cake puffs in the steamer and then deflates slightly as it stands." Lightly coat a nonstick 23 cm round cake pan with vegetable oil (if not nonstick, lightly butter pan and line the bottom with a round of buttered parchment or waxed paper). Put the pumpkin puree in the bowl of a food processor. Add the ricotta cheese, potato starch, sugar, and lemon zest and process until smooth. Scrape into a large bowl. In another large bowl, beat the egg whites to stiff peaks. Stir about one quarter of the whites into the pumpkin mixture to lighten it. Then gently fold in the remaining whtes. Scrape the mixture into the pan with a rubber spatula and smooth the top. Cover with aluminum foil and press the foil around the rim to seal. Put the pan on the steamer rack, place over simmering water, cover, and steam 20 minutes. Then set the lid ajar and steam 5 minutes more, or until the cake is set and a knife inserted into the center comes out clean. Let the cake cool, then refrigerate until chilled. To serve, invert a serving plate over the pan and quickly reverse the two to unmold the cake. Remove the parchment or waxed paper, if used. Cut the cake into wedges. Serve cold.