Combine the heavy cream and sugar in a medium-sized pot over medium heat. Stir occasionally until the sugar dissolves completely, about 5 minutes.
Once the sugar has dissolved, increase the heat to medium-high and bring the mixture to a gentle boil. Keep it boiling gently for 3 minutes, frequently stirring to prevent boiling over or scorching.
Remove the pot from the heat and cool for about 5 minutes.
Stir in the lemon juice and zest. Carefully pour the mixture into four serving dishes or ramekins. Allow the possets to cool to room temperature, then cover them with lids or parchment paper secured with rubber bands. Refrigerate the puddings for at least four hours (for best results, chill overnight). Garnish with fresh berries, mint leaves, or anything else you like before serving.
Notes
Watch for gentle, steady bubbling when bringing the pot to a boil, then start your timer.
Pour the mixture into jars or ramekins immediately—it thickens slightly as it cools and sets best when poured warm.
Let the jars cool at room temperature until they’re no longer warm to the touch before refrigerating to avoid condensation on the lids.
This is a great make-ahead dessert—chill for at least 4 hours, but overnight is even better to set fully.
Storage
Cover the jars with lids or parchment and refrigerate for up to three days. I don't recommend freezing, as it may separate.