Thinly slice beetroot (using a mandolin is easiest). Arrange in a baking dish lined with baking paper. Drizzle with 1Tbs vinegar, sprinkle with thyme leaves and season with Wild Porcini Salt.
Cover with foil and bake for 20 minutes or until just tender.
While the beetroot is roasting, heat oil in a frying pan over low heat. Cook onions and remaining vinegar for 20 minutes, stirring occasionally.
Brush each puff pastry sheet with butter and layer on top of each other. Lay into 20 x 30cm pan, folding in ends. Prick base with a fork. Bake for 5-10 minutes or until slightly baked.
Spread onion mixture over pastry. Top with beetroot slices and crumble over cheese. Finish with pine nuts. Bake for 20 minutes or until golden. Cut into pieces, serve and enjoy.