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 the sugar over the onions and continue cooking, stirring frequently, until the onions are golden brown and caramelized. This process can take about 20-25 minutes. Once done, stir in the fresh thyme leaves and season with salt and pepper. Set aside to cool.

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 tart pan, pressing it gently into the edges. Trim any excess pastry from the edges. Using a fork, prick the bottom of the pastry to prevent it from puffing up during baking.

Spread the caramelized onions evenly over the bottom of the puff pastry. Arrange the beefsteak tomato slices on top of the onions, slightly overlapping them. Sprinkle the grated Gruyère cheese over the tomatoes, if using. Brush the edges of the pastry with the beaten egg to create a golden, glossy finish.

Place the tart in the preheated oven and bake for 25-30 minutes, or until the pastry is golden brown and the tomatoes are tender. Remove from the oven and let it cool slightly before serving.