For the white chocolate layer, heat the cream in a saucepan until steaming and bubbles appear around the edge. Add the chocolate and lemon zest and stir until melted. Set six short tumblers tilted on their sides in a muffin tin (this is how you get a slanted layer). Pour the mixture into the glasses, then chill for 4 hrs, or until set.
For the berry layer, put all the frozen berries in a food processor and whizz until puréed. Push through a sieve using a wooden spoon directly into a jug, then stir through the lemon juice. Put the cream and sugar in a saucepan and warm gently until the sugar melts. Increase the heat and boil for 3 mins, stirring continuously. Remove from the heat and stir through the purée. Cool for 15 mins before sitting the glasses upright and pouring over the white chocolate layer. Chill for 4 hrs or until set.
Heat the oven to 170C/150C fan/gas 3. Line a baking sheet with non-stick parchment. To make the shortbread, put the butter and sugar in a bowl and beat with an electric whisk until pale and fluffy. Mix in ¼ tsp fine sea salt, the flour and pistachios to get a stiff dough, then bring together with your hands into a smooth ball. Put between two sheets of baking parchment and roll out to around ½cm. Chill for 20 mins. Cut into 20 rounds about 4cm and place on the baking sheet. Bake for 20-25 mins, then slide off the sheet onto a wire rack and leave to cool. Can be made two days ahead and kept in an airtight container.
When ready to serve, top the possets with the chopped pistachios and pomegranate seeds, and serve with the pistachio biscuits.