To prepare the pastry, add the buckwheat flour, ground almonds, arrowroot powder, flaxseed, salt and pepper to a mixing bowl. Stir together well.
Add the coconut oil and rub into the dough until well combined.
Add the water a little at a time, bringing the mixture together with your hands to form a dough. Add a little extra water if the mixture feels too dry. Wrap in silicone free baking paper and refrigerate whilst you prepare the filling.
Preheat the oven to 200°C, 180°C for fan assisted ovens.
Add the beetroots to a pan of boiling water. Reduce to a simmer and cook for 25 minutes until tender. Rinse under cold water and set aside until cool enough to handle.
Trim the tops and tips from the beetroot and peel the skin away.
Slice each beetroot into six wedges. Peel the onions and slice into 6 wedges so that they are roughly the same size as the beetroot.
Heat a tablespoon of coconut oil in an oven proof frying pan over a medium heat. Add the onion and beetroot and cook for a few minutes to soften the onion.
Add the garlic and thyme and cook for a minute more to soften the garlic.
Add the vinegar, coconut sugar and remaining coconut oil. Bring to a bubble. Season with salt and pepper and stir gently, turning the vegetables in the pan.
Allow the mixture to bubble for a few minutes until thickened and reduced to a syrup and the vegetables are starting to caramelise. Make sure the vegetables are packed together in an even layer with a slight gap around the edges of the pan. Set to one side to cool.
Roll the pastry out onto a floured surface - about a 3/4 cm thick.
Carefully flip or slide the pastry on top of the caramelised vegetables, tucking the pastry edges in at the sides, encasing the vegetables. Do not worry if the pastry tears, simply press it back together.
Transfer to the oven and bake for 30 minutes or until the pastry is crisp, golden and the vegetables caramelised.
Allow to cool in the pan then place a plate over the pastry and carefully flip the Tarte Tatin over to serve.