Start by making the pastry. In the bowl of a food processor add the flour and icing sugar and process to combine. Add the lemon zest and butter and pulse until the mixture resembles fine breadcrumbs. Add the egg yolk and tonic water and process until the mixture starts to come together. Remove from the food processor and wrap in plastic wrap and refrigerate for 30 minutes.
Preheat the oven to 180 celsius (355 Fahrenheit) and lightly grease 8 10cm loose bottom mini tart tins. Roll out the pastry until roughly 5mm thick and then cut circles from the pastry that are 2 cm larger in diameter than the tart tin. Line the each tart tin and trim excess. Chill for 15 minutes before lining with baking paper and filling with pastry weights. Bake for 10 minutes before removing the weights and baking for a further 5 minutes or until dry and lightly golden. Set aside to cool.
For the filling whisk together the eggs, caster sugar, lemon zest and juice. Add the cream and gin and whisk until combined. Carefully divide the filling between the mini tarts and bake for 7 to 10 minutes or until the filling is just set. Set aside to cool.
While the tarts are baking prepare the syrup. Place the caster sugar, tonic water and lemon juice in a saucepan over a low heat and stir to dissolve the sugar. Add the gin and juniper berries and zest and bring to the boil and reduce to a simmer for 10 minutes or until slightly thickened.
Serve the tarts with a dollop of cream and a drizzle of syrup.
Notes
1. Juniper berries are available in the spice aisle of supermarkets and delis. 2. I find the best way to trim the pastry for the tarts is to use a rolling pin and roll over the top of the tarts and the excess will come away easily.