Grease and line a 22 cm cake or pie dish with baking paper.
Heat 2 tablespoons of olive oil in a large frying pan on medium low and gently fry the onion until soft and beginning to brown. Add the thyme, sugar and balsamic vinegar. Cook for about 10 minutes until caramelised and rich. Remove from heat. Season with salt and pepper.
Drizzle a little olive oil in the bottom of the pie dish. Arrange the beetroot slices in a decorative pattern around the base.
Spread the beetroot slices with the caramelised onion layer.
Sprinkle the onion with ¾ of the crumbled Lemnos Traditional Fetta.
Top with the sheet of the puff pastry. Press it down firmly and tuck down or trim off any overhang.
Bake in the oven for ½ hour or until the pastry is golden brown and the beetroot is tender.
Remove from the oven, allow to cool slightly, loosen the edges with a knife and turn out quickly onto a serving plate.
Decorate with the remaining crumbled fetta and baby beetroot leaves if using them. Serve warm.