Caramelised onions:

Melt the butter in a large saucepan. Add the sliced onions and cook over low heat, tossing occasionally, for 1 hour or until onions turn soft, sweet, and golden brown.

Butternut squash puree:

Peel, de-seed, and dice the butternut squash into 1/2 – 1-inch cubes. (Alternately, you can purchase pre-cut butternut squash. See my notes above.)

Place diced butternut squash in a saucepan. Fill with water, completely covering the squash. Add the cinnamon stick and bring to a boil. Cook over medium heat until squash is fork tender, about 15 minutes.

Remove the cinnamon stick and drain the squash in a fine mesh strainer.

Add squash to a food processor or high-speed blender and purée until smooth.

Season with salt and pepper to taste. Refrigerate until ready to use.

Tart:

Preheat oven to 400° F / 205° C. Line a half-sheet rimmed tray with parchment.

Heat the olive oil in a large saucepan. Add the butternut squash and cook, turning periodically with a spatula, until golden brown and just barely tender, about 8-10 minutes.

Add the spinach and sautee until wilted, about 2-3 minutes more. Season with salt, pepper, and chili flakes. Set aside to cool.

Meanwhile, unroll puff pastry on prepared tray.

Spread with the butternut squash puree, allowing a 1-inch border around the edges.

Scatter with the caramelised onions, followed by the butternut squash and spinach mixture.

Top with blue cheese and half of the Parmesan.

With a pastry brush, brush egg wash along the pastry border, then turn up edges and crimp with a fork. .

Brush crimped edge with more egg wash.

Bake in preheateed oven for 25-30 minutes, or until pastry turns deeply golden brown.

Sprinkle with chopped rosemary and remaining Parmesan. Season with salt and pepper to taste. Serve warm.