Cut the squash in 2 parts, scrape out the seeds and the fiber.
Spray the baking sheet with oil and place the squash on the baking sheet cut side down.
Bake for about 30 minutes, remove from the oven and cool: flip the squash so that cut side faces up – that will speed up the cooling.
Once the squash is cooled, peel it. The skin will come off really easily.
Then, using a large grater, grate the squash and put it in a large mixing bowl.
In the mean-time, finely dice an onion, heat olive oil in the frying pan and saute diced onions until translucent.
Add cooked onion to the mixing bowl with squash. Mix onions and squash well, add salt and pepper to taste.
How to fill puff pastry with prepared butternut squash filling:
Dust your working surface with flour and cut one sheet of puff pastry into 9 equal squares.
Take each square of puff pastry, stretch it a bit to make it bigger, place it on floured work surface, fill one half of the square with butternut squash mixture, fold the other half on top to form a triangle, press the sides to close with your fingers.
Whisk an egg in a small bowl, until mixed well and brush the top of each turnover with a whisked egg.
Transfer turnovers to the lightly greased cookie sheet. Sprinkle the top of each turnover with sesame seeds.
Bake for about 20 minutes or until nice golden color.
I have step-by-step photos above the recipe box. Make sure to scroll up to see those.