Place all pastry ingredients in a food processor and pulse until the mixture resembles fine breadcrumbs.

Gradually trickle in ice cold water while pulsing until the mixture clumps together.

Press the dough into a disc without kneading, wrap in cling film, and chill in the fridge for 30-60 minutes. Preheat the oven to 180° C / 355° F.

Roll out the chilled pastry onto a lightly floured surface until about 2-3 mm thick.

Line a tart dish with the rolled pastry and pierce the bottom with a fork.

Line with baking paper and fill with baking beads. Bake for 15 minutes, then remove the paper and beads and bake for an additional 5-10 minutes until the bottoms are set.

For the custard filling, dissolve cornstarch in 120 ml of plant milk in a bowl.

Heat the remaining plant milk in a pot, whisk in sugar until dissolved.

While whisking, add the cornstarch mixture and heat until thickened. Whisk in salt, vanilla, cardamom, and vegan butter until smooth.

If using rhubarb, trim and cut into 3 cm pieces, arrange in a baking tray, add water, and sprinkle with sugar. Bake at 190° C / 375° F for 15-20 minutes until tender.

Fill the baked pastry with the custard and smooth the top. Bake at 150° C / 300° F for about 35 minutes until set but wobbly in the middle.

Allow to cool completely, then refrigerate for at least 2 hours before serving. Top with rhubarb or fruit compote if desired.