Zest the mandarins and slice them in half.
Juice the mandarin halves using a reamer.
Scoop out any remaining flesh from the mandarins.
Add heavy cream, sugar, salt, zest, and turmeric (if using) to a saucepan over medium heat.
Bring to a boil, then reduce heat slightly and let simmer for 5-6 minutes, stirring often.
Strain through a fine mesh sieve and discard the zest.
Whisk in the mandarin juice until the mixture thickens slightly.
Arrange the mandarin halves on a plate or platter and pour in the posset mixture.
Refrigerate for at least 3 hours or up to overnight before serving.