Preheat the oven to 200°C.
Rub the butter into the flour and icing sugar until the mixture resembles fine breadcrumbs.
Mix the egg, lemon juice, and 30ml of water, then incorporate into the flour to form a dough.
Knead the dough into a ball and chill in the fridge for 15 minutes.
Roll the pastry to 3mm thickness and line a 23cm tart tin.
Blind bake with parchment and baking beans for 15 minutes.
Remove beans and bake for another 8 minutes until dry.
Trim excess pastry and reduce oven temperature to 180°C.
Blend the beetroot and cream into a thick purée using a food processor.
Whisk eggs and sugar together, then stir in the beetroot purée, spices, and lemon zest.
Pour filling into the case and bake for 30–40 minutes until set with a slight wobble.