Heat the olive oil and butter in a large skillet over medium heat. Add the thinly sliced onions and cook, stirring occasionally, until they begin to soften. Sprinkle with sugar and a pinch of salt, then reduce the heat to low. Continue cooking, stirring occasionally, until the onions are golden brown and caramelized. This process can take up to 30 minutes. Stir in the fresh thyme leaves and set aside.

Preheat your oven to 375°F (190°C). Roll out the thawed puff pastry on a lightly floured surface to fit your tart pan. Transfer the pastry to the pan and trim any excess edges. Prick the base of the pastry with a fork to prevent it from puffing up too much during baking.

Spread a layer of ricotta cheese evenly over the puff pastry base. Arrange the sliced beefsteak tomatoes on top of the ricotta, slightly overlapping them. Spoon the caramelized onions over the tomatoes, ensuring they are evenly distributed.

Brush the edges of the puff pastry with the beaten egg to create a golden finish. Bake in the preheated oven for 25-30 minutes, or until the pastry is golden brown and crisp.

Allow the tart to cool slightly before slicing. Serve warm or at room temperature, garnished with additional fresh thyme leaves if desired.