Preparation:

Heat the oven to 200°C. Clean the beetroot thoroughly and cut it into smaller quarters. Put the pieces in a baking dish, drizzle with olive oil and bake for about 30 minutes. Add the chopped onion and cook for another 5-10 minutes. Let it cool down a bit.
Roll out the puff pastry into a large rectangle and make a 1 cm border on its edge. Put on a baking sheet and rub the edge. We hatch the center with a sharp knife, but do not cut it completely. Drizzle the beetroot with balsamic and put it on the cake together with the onion, feta and thyme leaves. Season the top well. Bake for about 20-30 minutes, or until golden.