Preheat your oven to 175°C (350°F). Lightly grease the bottom and sides of a 14" rectangular tart tin with some cooking oil and line the base with a sheet of parchment paper.
Make a "flax egg" by adding the ground flaxseeds to a bowl with 2.5 tablespoons water. Stir and allow to sit for at least 5 minutes.
In your food processor, combine the flour, ground almonds, vegan butter, coconut sugar, salt, and flax egg until the ingredients stick together to form a dough.
Evenly press the dough into the base and sides of the tart tin and use a fork to prick several holes into the base. Bake the crust for 12-15 minutes then allow it to cool (do not remove the tart from the tin). Leave the oven on for the rhubarb.
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 lay out the rhubarb stalks. Coat them in the caster sugar and cover the dish with a lid or some extra parchment paper. Bake for 25-30 minutes and allow to cool completely. Reserve the syrup for serving.
For the custard first add the soya milk, cornstarch, and agar-agar to a saucepan and whisk well. Add in the remaining ingredients and place on medium heat until simmering. Vigorously whisk the mixture over medium heat for 8-10 minutes until the mixture has thickened.
Allow the custard to cool slightly for 5 minutes, whisk well again, and pour the custard into the cooled tart crust. Allow the filling to come to room temperature for 10-15 minutes and place the tart in the fridge to chill for at least 2 hours or until set.
Arrange the rhubarb strips on top of the tart. Serve the tart with some of the reserved rhubarb syrup.
Store in the fridge in an airtight container and consume within 2 days.