In a large bowl, mix all ingredients with a wooden spoon or with your hands until well combined.
Transfer to a working surface and continue kneading until a smooth dough forms.
Flatten dough into a thick disc, cover and refrigerate for 30 minutes.
Preheat oven to 200°C
Roll out evenly on a floured surface, about 5mm thick, then transfer into a lightly greased pie dish. Shape the edges as you like and prick the bottom a few times with a fork.
Bake the crust for about 20 minutes or until the crust becomes golden brown. Place baking paper with raw rice grains (or other grains) in the middle of the pie so the dough doesn’t puff up as it bakes.
Method for the filling
Peel and slice the rhubarb in large pieces. In a pot, cook the rhubarb, water, and sugar together on medium heat for about 40 minutes or until obtaining a rhubarb compote. You should have the same consistency as applesauce, except with rhubarb "strings".
Then, blend the rhubarb compote with the applesauce and beetroot until smooth. You can add a little more beetroot if you want a brighter pink.
In a saucepan, heat up the rhubarb mixture and slowly add maïzena and agar agar powder while whisking. Cook on medium heat for about 10 minutes and continue whisking until smooth.
Add the rhubarb mixture to the crust and spread it out evenly. Leave to cool in the fridge for at least 2 hours or until set. Serve.
Notes
This tart is best when fresh, though leftovers can be kept covered and refrigerated for up to a week. I doubt you will wait that long to eat it though.