Caramelize the Onions: In a large skillet, heat the olive oil over medium heat. Add the sliced onions, and cook for about 10 minutes, stirring occasionally until they begin to soften.
Add Sugar and Balsamic: Sprinkle in the sugar and balsamic vinegar. Continue cooking for another 15-20 minutes, stirring frequently until the onions are deep golden brown and caramelized. Season with salt and pepper, then set aside.
Preheat the Oven: Preheat your oven to 400°F (200°C).
Prepare the Flatbread: If using pizza dough, roll it out into your preferred flatbread shape. Place it on a baking sheet lined with parchment paper.
Assemble the Flatbread: Spread the caramelized onions evenly over the flatbread, leaving a small border around the edges. Sprinkle the grated Gruyère cheese on top, and then add the thyme.
Bake the Flatbread: Bake in the preheated oven for about 12-15 minutes or until the cheese is bubbly and golden.
Finish and Serve: Remove from the oven, allow to cool for a few minutes, then slice into pieces. Optionally, top with fresh arugula for a peppery contrast.
Notes
Optionally garnish with fresh arugula for added flavor.