01 - Pour heavy cream and granulated sugar into a medium saucepan. Place over medium heat, stirring gently until sugar completely dissolves.
02 - Continue heating until the cream just begins to simmer. Watch carefully to prevent boiling. Remove from heat immediately when bubbles appear around the edges.
03 - Stir in fresh lemon juice and grated lemon zest. The acid will react with the cream, causing the mixture to thicken noticeably.
04 - Let mixture sit at room temperature for 10 minutes to cool slightly while thickening further.
05 - Distribute the mixture evenly among 4 individual serving glasses or ramekins, leaving space at the top for garnishes if desired.
06 - Refrigerate for at least 3 hours until fully set with a silky, pudding-like consistency. The posset will firm up as it cools.
07 - Serve chilled. Garnish with fresh berries, additional lemon zest, or a shortbread cookie alongside if desired.