Preheat oven to 425 degrees F.

Line a baking sheet with parchment paper. Place the butternut squash whole on the baking sheet.

Roast the squash whole for 30 minutes.

In the meantime, peel and cut the onion into small pieces.

Melt one tablespoon of butter with the olive oil in a frying pan.

Add the onions. Cook over a low flame, stirring frequently, until caramelized.

Remove the onions from the pan. Set aside.

Add the remaining tablespoons of olive oil to the pan.

Add the sage leaves to the pan and toss to coat.

Sprinkle the sugar over the top of the leaves. Stir frequently until they become crispy.

Remove from the heat and set aside.

Remove the squash from the oven and carefully slice each squash in half.

Replace the halves on the baking sheet.

Bake for 20-30 minutes or until you can run a knife easily through the thickest part of the squash pieces.

When the squash halves are cool enough to handle, scrape out the seeds.

Discard or save the seeds to roast later.

Scoop the butternut squash flesh out into a casserole dish.

Mix in the caramelized onions.

Cut the remaining two tablespoons of butter into pieces and mash into the warm squash.

Season with kosher salt and freshly ground pepper to taste

Cover the dish with aluminum foil and place it in the oven to keep warm.

Remove from the oven, top with the candied sage and pistachios, and serve.