Pre-heat the oven to 190°C/375°F
Melt the butter and oil over medium heat in an oven proof frying pan. Add the beets and onions toss to coat in the butter and oil. Ensure the beetroot and onions are snug in the frying pan.
Add the balsamic vinegar, brown sugar, rosemary, thyme, salt and pepper and stir well, covering the beets and onion.
Cook for about 20 minutes, constantly turning to coat the beets and onions. During this time, add the sherry and add balsamic and sugar if required.
Cover with foil and place into a preheated oven for 30-40 minutes until beets are soft and onions caramelized.
Remove from the oven and arrange neatly, largest cut side down.
Cut the puff pastry the size of the frying pan and cover the beets and onion, tucking in the edges of the pastry in the pan.
Bake for 20-25 minutes, until pastry is golden brown.
Allow the Tarte Tatin to cool for about 10 minutes. Place a large plate over the pan, and quickly flip it over, removing the pan. Pour any juices over the plate.
Drizzle balsamic glaze over the Tarte Tatin and crumble goats cheese over it. Garnish with fresh thyme.