Onions:

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

Butternut Squash Purée:

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 fork tender, about 15 minutes.

Remove the cinnamon stick and pass squash through a fine mesh strainer to drain.

In a food processor or high-speed blender, purée squash 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 sauté 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.

With a small stepped palette knife, spread pastry with butternut squash purée, allowing a 1-inch border around the outer edge.

Scatter with the 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 edges with more of the beaten egg.

Bake 25-30 minutes, or until deeply golden brown.

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