If you are using frozen puff pastry ensure it is properly thawed first
Preheat the oven to 180C.
Peel and finely chop the onion (half moons work well) and garlic. Roughly chop the mushrooms into slices.
Roughly chop the mushrooms into slices.Put a large wide bottomed frying pan on a medium heat and leave to warm up. Once it is hot, add 1 tbsp oil along with the onions, garlic and mushroom. Cook the veg for about 10-15 minutes stirring regularly until they start to char.
In a bowl mix together the balsamic vinegar, maple syrup, tamari and whisk it together using a fork. Add to the veg and leave to reduce for a further 3 minutes.
Remove from the heat and allow to cool slightly. Take the thyme leaves from the sprigs and add half of them to the veg and mix through.Line a medium sized baking tray (approximately the size of the sheet of puff pastry) with baking parchment.
Transfer the cooked veg and spread out evenly. Fit the sheet of puff pastry to fit the size of the baking tray and carefully place on top tucking in any edges (if your tray is smaller than the puff pastry sheet you will need to cut it slightly)
Put in the preheated oven and bake for 20-30 minutes until the pastry starts to golden up.
Remove the dish from the oven, put a board that is slightly bigger than the pan on top of it and invert so that the pastry is now on the bottom and the cooked veg on top. Garnish with the remaining thyme leaves, slice and enjoy!