Most puddings use eggs or cornstarch for thickening, but this classic British dessert uses lemon juice, which makes it even easier to make.
In a medium saucepan, bring the cream, sugar, and lemon zest to a boil, stirring until the sugar dissolves, 8 to 10 minutes. Remove from heat. Stir in the lemon juice. Let cool slightly, about 20 minutes.
Strain the cream into a medium bowl or small pitcher. Divide evenly among six 4-oz. ramekins or bowls. Refrigerate, uncovered, until set, about 3 hours. Let sit for 10 minutes at room temperature. Top the possets with the blueberries.