Grease a 8-inch tart tin with a removable bottom with coconut oil. In a food processor combine oats, buckwheat groat, almonds and process until coarsely ground. Add dates, coconut oil and maple syrup, pulse until mixture looks like wet sand.
Firmly press the mixture into the bottom and sides of the prepared tart tin. Place in the freezer to set while you work with your filling.
Pour the coconut milk, soy milk into a saucepan and place on the stove on low heat. When the mixture becomes warm, add the cornstarch, agar-agar, maple syrup and vanilla. Stirring constantly, cook until agar and cornstarch completely dissolve. Allow to boil for approximately 30 to 60 seconds. Pour mixture into tart and refrigerate at least 2 hours or overnight to set.
Make the plum jelly; Add plums in a blender. Blend into a puree. Add water and blend it again.
Strain juice through a mesh sieve.
In a small pot, bring the plums to a boil. Add agar-agar and maple syrup. Stirring constantly, cook until agar-agar completely dissolve. Allow to boil for approximately 30 to 60 seconds. Turn off the heat and allow the mixture to cool to room temperature before gently pouring over the top of the set vanilla filling.
Place tart in the fridge for 2 hours to set. Remove tart from the tart tin and garnish with more sliced plums. Enjoy!