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Preheat the oven to 375°F (190°C).
Heat the olive oil and butter in a large, oven-safe skillet over medium heat. Add the sliced onions and cook, stirring occasionally, for about 20 minutes until they soften and begin to caramelize.
Add the brown sugar and thyme, stirring to combine. Continue to cook for an additional 5-10 minutes until the onions are deeply caramelized.
Pour in the white wine and balsamic vinegar, scraping up any bits stuck to the pan. Cook for a few more minutes until the liquid reduces by half and the onions are tender and caramelized. Season with salt and pepper to taste.
Roll out the puff pastry on a lightly floured surface and cut it into a circle that is slightly larger than the skillet.
Carefully place the puff pastry over the caramelized onions in the skillet, tucking the edges down around the onions.
Bake in the preheated oven for 25-30 minutes, or until the pastry is golden and puffed.
Remove from the oven and let it cool for a few minutes. Carefully invert the skillet onto a large plate to flip the tart, so the onions are on top.
Garnish with fresh thyme sprigs, if desired, and serve warm.