For the posset: In a medium saucepan over medium heat, bring the cream, sugar and lemon peel to a gentle simmer, stirring frequently to make sure all the sugar dissolves. Remove from the heat and stir in the lemon juice; you will see the mixture start to thicken. Strain the mixture through a sieve into a heat-proof pouring jug, allow to cool for 5 minutes, and then pour into your chosen serving glasses. (I like to serve mine in vintage champagne coupes.) Refrigerate until set, at least 5 hours.
For the candied lemon peel: Place the lemon peel and 2 cups water in a small saucepan and bring to a boil; strain, reserving the peel. In another saucepan, combine the sugar with 2 cups water. Bring to a simmer to dissolve the sugar. Add the lemon peel to the syrup and simmer for 20 minutes. Leave the peel in the sugar syrup to cool, then transfer to a wire rack to dry.
Garnish the posset with candied lemon peel if using, or serve it on its own or with some macarons or shortbread.