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 25-30 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 bottom of the pastry with a fork to prevent it from puffing up too much during baking.

Spread the caramelized onions evenly over the puff pastry. Arrange the beefsteak tomato slices on top of the onions, slightly overlapping them. Sprinkle with salt, pepper, and grated Gruyère cheese if using.

Brush the edges of the puff 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. Garnish with additional fresh thyme leaves if desired. Serve warm or at room temperature.