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 continue cooking, stirring frequently, until the onions are golden brown and caramelized. This process should take about 20-25 minutes. Stir in the 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 or baking sheet. 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. Top with the caramelized onions, spreading them out in an even layer. Arrange the beefsteak tomato slices on top of the onions, slightly overlapping them. Season the tomatoes with salt and pepper.

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

Allow the tart to cool for a few minutes before slicing and serving. Garnish with additional fresh thyme leaves if desired.