01 - Pour heavy cream and granulated sugar into a medium saucepan. Place over medium heat, stirring gently until sugar completely dissolves.
02 - Continue heating the mixture until it just begins to simmer, watching carefully to prevent boiling. Remove from heat immediately when bubbles form around the edges.
03 - Stir in fresh lemon juice and grated lemon zest. The acid will cause the cream to thicken noticeably as you mix.
04 - Let the mixture sit at room temperature for 10 minutes to cool slightly while thickening further.
05 - Distribute the posset evenly among 4 individual ramekins or dessert glasses, wiping any drips from the rims.
06 - Refrigerate for at least 3 hours until the texture becomes firm and silky, similar to pudding.
07 - Serve chilled with optional garnishes of fresh berries, additional lemon zest, or shortbread cookies alongside.