Preheat the oven to 220C/425F/Gas 7.
For the posset, mix together the cream, milk, sugar and lemon juice in a clean bowl, then spoon the mixture into a tall dessert glass. Garnish with the lemon slice and place in the fridge to chill.
For the shortbread, place in the food processor the butter, flour and sugar and blend together. Then roll out the mixture directly onto a baking sheet and place into the oven to bake for 5-10 minutes, or until lightly golden and crisp.
To serve, cut up the shortbread and place onto a plate alongside the lemon posset.