For the crust, add the roasted buckwheat to your food processor and blitz for a couple of minutes until roughly ground. Add the cashews and blitz for another minute or so until the texture resembles a light crumb consistency. Add the remaining base ingredients and continue to process until all the ingredients start to stick together to form a dough consistency.
Take a piece of parchment paper and use it to rub the inside of the tin with coconut oil. Sprinkle with a layer caster or coconut sugar. This will prevent the tart from sticking to the edges of the tin! Press the mixture evenly into your tart tin, making sure all the edges are evenly covered.
Set the base aside in the freezer to set.
For the custard first create your slurry by adding the agar agar, turmeric, and 2 tbsp water to a jar and stir to remove any lumps. Add the soy milk, soy yogurt, cornstarch, vanilla extract, and coconut sugar along with your “slurry” mixture to a heavy-bottommed pan and bring to the bowl, whisking continuously to avoid the mixture sticking to the bottom of the pan.
Lower the heat and allow this custard to simmer and cook out for about 2 minutes, ensuring that you keep stirring.
Remove the custard from the heat and allow it to cool slightly for about 5 minutes before pouring it into the tart tin. Be careful not to overfill, any remaining custard and be stored in an airtight container in your fridge (or just eaten on the spot!).
Allow the tart to cool for about 15 minutes before placing it in the fridge to set for 1-2 hours.
Meanwhile, to prepare the rhubarb, heat your oven to 180°C. Wash the rhubarb stalks and cut them into strips to fit horizontally across the top of your tart.
Line a baking dish with some parchment paper and arrange the rhubarb stalks. Coat them in the caster sugar and cover the dish with a lid or some extra parchment paper.
Bake for 15-20 minutes and allow to cool completely before arranging the rhubarb strips on top of the tart.