Make the frozen yogurt:
Place the Greek yogurt in a large mixing bowl. Add the finely grated lemon zest and stir until well combined.
Roughly chop the pistachios and stir them into the yogurt. Add the honey and double cream, mixing until evenly combined.
Break the halva into approximately 1 cm pieces and gently fold them into the mixture.
Transfer the mixture to a freezer-safe container with a lid and freeze for at least 4-5 hours. For a smoother, creamier texture, stir the mixture thoroughly every hour during the first 3 hours of freezing. If using an ice cream maker, follow the manufacturer's instructions.
Make the cinnamon wafers:
Cut twelve 10 cm squares from the filo pastry.
Stack six sheets, brushing each layer lightly with melted butter and sprinkling with a little caster sugar and ground cinnamon before adding the next sheet. Do not butter the top layer; simply sprinkle it with sugar and cinnamon.
Repeat with the remaining six sheets.
Using a 10 cm round pastry cutter or the rim of a glass, cut out circular wafers.
Preheat the oven to 200°C (180°C fan/Gas Mark 6). Place the wafers on a baking tray lined with baking paper and bake for 6-8 minutes, or until golden brown and crisp.
Transfer them to a wire rack and let them cool completely.
To serve:
Remove the frozen yogurt from the freezer for 5-10 minutes before serving to make scooping easier.
Scoop the frozen yogurt into balls and sandwich each scoop between two crisp cinnamon wafers.
Finish with a light dusting of icing sugar and pinch of chopped pistachios for an elegant presentation.