In a medium (deep) saucepan, combine the cream, sugar and lemon zest. Bring to a boil over medium heat and cook, stirring frequently, to dissolve the sugar. Don't walk away from the mixture as it can easily boil over. If it gets close to the top, remove the pan briefly from the heat to slow the boil.
Cook until the mixture reduces to exactly two cups. (SEE NOTE) Remove from the heat and add the lemon juice. Set the saucepan aside to cool until a skin forms on top, about 20 minutes. Strain the posset through a fine mesh strainer into a bowl. Discard the lemon zest.
Divide the mixture between 6 individual bowls or serving glasses. Refrigerate, uncovered, until set, about 3 hours. Once fully chilled, cover with plastic wrap and refrigerate for up to 2 days.
To serve, unwrap and allow the dessert to rest at room temperature for 10 minutes before serving. Garnish with fruit blueberries or raspberries and serve.