Preheat oven to 400°F (200°C). Toss the cubed butternut squash with olive oil, salt, and pepper. Spread on a baking sheet and roast for 25-30 minutes until golden and tender.

While squash roasts, caramelize the onions. Heat olive oil in a pan over medium-low heat. Add sliced onions and cook, stirring often, for 30-40 minutes until golden brown and soft.

Roll out the puff pastry on a parchment-lined baking sheet. Score a 1-inch border around the edge and prick the center with a fork. Pre-bake for 10 minutes.

Spread caramelized onions over the pre-baked puff pastry. Top with roasted squash, crumbled goat cheese, and sprinkle with thyme.

Brush edges with egg wash if using. Bake for 15-20 minutes until the pastry is golden and crisp.

Cool slightly before slicing. Serve warm or at room temperature.