To obtain the lemon juice, cut 4 lemons in half lengthwise. With a spoon or a knife, remove the pulp of the lemons and then squeeze them into a bowl, discarding the seeds, to make 5 tablespoons of juice. The lemon shells will be used as the little glasses where the filling will go.
Separately, in a medium saucepan add the whipping cream with the sugar and lemon zest. Bring to medium heat and cook for about 3-5 minutes, stirring constantly with a rubber spatula, until the sugar dissolves and the mixture begins to boil.
Reduce the heat to medium-low and cook for 3-5 minutes more, stirring constantly, until the mixture begins to thicken. Remove the pan from the heat, add the lemon juice and stir very well to integrate it into the mixture.
Using a fine mesh strainer, strain the mixture directly into a measuring cup and let it cool for 20 minutes.
Place the halves of the lemons that will be used as drinking glasses on a plate or tray to give them stability. Pour the mixture into these lemons filling each one almost completely. If there is leftover mixture, it can also be placed in small glasses or ramekins.
Refrigerate the lemon possets uncovered until set, at least 3 hours or overnight. Serve with whipped cream and berries if desired.