Heat the olive oil and butter in a large skillet over medium heat. Add the sliced onions and cook, stirring occasionally, until they are soft and golden brown, about 20-25 minutes. Stir in the balsamic vinegar and cook for another 2 minutes. Season with salt and pepper, then set aside to cool.

Preheat your oven to 400°F (200°C). Roll out the puff pastry on a lightly floured surface and transfer it to a baking sheet lined with parchment paper. Score a 1-inch border around the edges of the pastry using a sharp knife. Prick the center of the pastry with a fork to prevent it from puffing up too much during baking.

Spread the caramelized onions evenly over the center of the puff pastry, staying within the scored border. Arrange the sliced beefsteak tomatoes on top of the onions, overlapping slightly. Sprinkle the fresh thyme leaves over the tomatoes and season with salt and pepper. If using, sprinkle the grated Gruyère cheese over the top.

Bake in the preheated oven for 20-25 minutes, or until the pastry is golden brown and the tomatoes are tender. Allow the tart to cool slightly before slicing and serving.