Steps
Preheat your oven to 400°F (200°C).
Slice the onion thinly.
In a skillet, heat the olive oil over medium heat and add the sliced onions.
Cook the onions, stirring occasionally, for about 15-20 minutes until they are golden brown and caramelized.
While the onions are caramelizing, roll out the puff pastry on a parchment-lined baking sheet.
Once the onions are done, spread them evenly over the pastry, leaving a small border around the edges.
Crumble the goat cheese over the caramelized onions and sprinkle with fresh thyme, salt, and pepper.
Beat the egg and brush it over the edges of the pastry for a golden finish.
Bake the tart in the preheated oven for 20-25 minutes, or until the pastry is golden and puffed.
Remove from the oven and let it cool slightly before slicing and serving.