Halve 4 lemons lengthwise. Remove pulp with a spoon (a small paring knife can help to get it started) and set aside.
Combine heavy cream and sugar in a small pot over low heat. Simmer just until sugar is dissolved, 2 to 3 minutes. Stir in lemon juice.
Pour cream mixture into the empty lemon halves. Refrigerate until set, 2 to 3 hours.
Garnish lemon possets with 1 raspberry or a few blueberries and a mint leaf. Serve cold.
