In a large pot, heat the butter and olive oil. Add sliced onions and cook until soft and just turning golden, stirring frequently. Add the Caramelised Balsamic Vinegar and stir well. Cook for a further 5 minutes, add the thyme leaves, season and cook for 1 minute. Remove from heat and cool to room temperature. Preheat oven to 200 degrees Celsius, slice one sheet of pastry in half and cover one side with Dukkah, place the other side on top and roll out to previous size, roughly 5 mm thick. Grease a pastry-baking dish well and cover with pastry, cutting off any excess. Top pastry centre with onion mixture, cover evenly and crumble over feta cheese. Prick with a fork all around the edges and egg wash edges. Bake for 25 minutes or until pastry has turned golden. Serve warm with a green salad.