Preheat oven to 400°F.
In a large bowl, combine butternut squash cubes, olive oil, ½ teaspoon salt and pepper to taste.
Transfer to a large rimmed baking sheet.
Bake in preheated oven for 25 minutes, until tender.
In a large skillet, add 3 tablespoons butter over medium heat.
Add sliced leeks and chopped sage and season with ½ teaspoon salt and pepper to taste.
Sauté until the leeks are tender, about 10 minutes, stirring frequently.
Grease 9x13" baking dish with cooking spray or oil.
Combine the leek mixture and roasted butternut squash in baking dish. Give it a little stir to mix them up a bit.
Sprinkle with goat cheese and hazelnuts evenly.
Drizzle with heavy cream.
Bake at 375°F for 30-35 minutes, until the gratin is heated and bubbling.
Serve and enjoy!