Heat olive oil in a skillet over medium heat. Add sliced red onions and cook, stirring occasionally, until caramelized, about 20 minutes.

Preheat the oven to 375°F (190°C). Roll out the puff pastry and fit it into a tart pan. Prick the bottom with a fork.

In a mixing bowl, whisk together eggs, heavy cream, thyme, salt, and pepper. Stir in the caramelized onions and goat cheese.

Pour the mixture into the prepared pastry shell.

Bake in the preheated oven for 25-30 minutes, or until the tart is set and golden brown.

Let cool slightly before slicing and serving.