Heat olive oil and butter in a skillet over medium-low heat. Add sliced onions, salt, and sugar (if desired). Cook for 20-30 minutes until golden brown.

Preheat the oven to 375°F (190°C). Roll out puff pastry to fit tart shells, cut and line them. Brush edges with beaten egg.

Combine caramelized onions, goat cheese, heavy cream, thyme, and pepper in a bowl until well mixed to create filling.

Spoon the filling into tart shells, ensuring even distribution without overfilling.

Bake in the preheated oven for 15-20 minutes until golden brown and bubbly.

Let cool slightly before serving. Drizzle with honey if desired.