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, about 30-40 minutes. Stir in the thyme leaves and set aside.

Preheat your oven to 400°F (200°C). Roll out the thawed puff pastry on a lightly floured surface to fit your tart pan or baking sheet. Transfer the pastry to the pan and trim any excess edges. Prick the pastry all over with a fork to prevent it from puffing up too much during baking.

Spread the caramelized onions evenly over the puff pastry, leaving a small border around the edges. Arrange the sliced beefsteak tomatoes on top of the onions, overlapping slightly. Sprinkle with salt, pepper, and the grated Gruyère cheese if using. Fold the edges of the pastry inward to create a rustic crust.

Brush the edges of the pastry with the beaten egg for a golden finish. Bake in the preheated oven for 25-30 minutes, or until the pastry is golden brown and the tomatoes are tender. Allow the tart to cool slightly before slicing and serving.