To begin, zest some clementines until you have about 1 heaping teaspoon of zest. Set the zest aside.
Cut 5 clementines in half, then scoop the fruit out. I recommend using a serrated grapefruit spoon for this! Set the fruit aside in a little bowl. Repeat until you have 10 half-clementine shells. Arrange them all on a plate and set them aside for later.
Take the scooped out fruit and press it through a strainer until you have 37 grams, or about 3 tablespoons, of juice. Set aside.
Now, add the granulated sugar and clementine zest to a mixing bowl. Using your fingers, rub the clementine zest into the sugar for about 1-2 minutes. The sugar should become fragrant and smell very clementine-y.
Add the clementine zest sugar and heavy cream to a small saucepot. Bring the mixture to a boil, whisking frequently, then reduce to a simmer. Simmer for about 6-8 minutes, whisking frequently to prevent the mixture from bubbling over.
When done, remove the mixture from heat. Pour in the reserved clementine juice, and whisk until completely incorporated. Then, pour the creamy mixture through a mesh strainer to remove any pieces of zest. I recommend pouring the mixture into a vessel with a pouring lip, like a pyrex measuring cup. This will make it easier for your to pour the mixture into the hollowed-out clementines!
Carefully pour the creamy mixture into the hollowed-out clementines. (If you have any mixture left over, pour it into a small bowl or ramekin.)
Put the plate of possets into the refrigerator for AT LEAST 2 hours, then either serve them or cover them in the fridge for up to 3 days.
When ready to serve your possets, top each with a dollop of whipped cream and some fresh clementine zest (optional).
Enjoy!!