Preheat the oven to 200°C.
Press the pastry sheet into a 20cm sandwich tin, leaving an overlap over the edge of the tin. Be careful to press firmly into the edges. Place into the fridge for 10 minutes.
Line the pastry with a square of non-stick baking paper and pour in some baking beans. Bake for 10 minutes. Remove the paper and baking beans, and bake for a further 12 minutes — until lightly golden.
Remove from the oven and lower the temperature to 150°C. Using a sharp knife, trim the edges of the pastry in line with the top of the tin. Leave to one side.
To a mixing bowl, add the eggs, sweetener, lemon juice, and lemon zest, reserving 1 tsp of zest for later. Whisk with a balloon whisk, until fully combined.
Add the cream alternative and milk to a small saucepan and set on a low-medium heat. Heat until just simmering — do not boil. Pour into the egg mixture very slowly, continuing to whisk the egg mixture as you pour in the hot cream and milk.
Place the cake tin onto a baking tray and set on a shelf in your oven. Pour the egg and cream mixture into the pastry case and bake for 35 minutes.
The custard should be set round the edges, but still have a slight wobble in the centre. Leave to cool completely in the tin.
Remove the tart from the tin and transfer to a plate. Place the lemon slices and blackberries around the edge of the tart, then sprinkle over the reserved 1 tsp lemon zest. Slice and serve!