Preheat your oven to 400°F (200°C).

In a large bowl, combine the cubed butternut squash with 1 tablespoon of olive oil, ½ teaspoon of salt, and ½ teaspoon of black pepper. Spread the cubed squash on a baking sheet and roast it in the oven for 20-25 minutes until tender.

While the squash roasts, heat the remaining olive oil in a skillet over medium heat. Add the sliced onions, a pinch of salt, and thyme. Cook until caramelized and golden brown, about 15-20 minutes.

Roll out the puff pastry on a floured surface to fit your tart pan. Transfer the pastry into the pan and trim any excess edges.

Spread the roasted butternut squash evenly on the pastry, followed by the caramelized onions. Sprinkle the grated cheese on top.

Beat the egg in a small bowl and brush it over the pastry edges.

Place the tart in the preheated oven and bake for 25-30 minutes, or until the pastry is puffed and golden.

Once out of the oven, let it cool for a few minutes. You can garnish it with fresh herbs if desired. Slice and serve warm.