Preheat your oven to 200°C (392°F). Line a baking sheet with parchment paper.

Peel the butternut squash and cut it in half lengthwise. Remove the seeds. Place each half cut side down on a cutting board. Using a sharp knife, make thin slices across the squash, being careful not to cut all the way through. The slices should be about 3 mm apart.

Place the prepared squash halves on the lined baking sheet. Drizzle with olive oil and season with salt and pepper. Rub the oil and seasoning evenly over the squash.

Roast in the preheated oven for 45-50 minutes, or until the squash is tender and the edges are golden and slightly crispy.

While the squash is roasting, cook the bacon in a skillet over medium heat until crispy. Remove from the skillet and place on a paper towel to drain. Once cooled, chop the bacon into small pieces.

In a small bowl, combine the chopped bacon, chopped rosemary, and hazelnuts.

Remove the squash from the oven and sprinkle the bacon, rosemary, and hazelnut mixture over the top. Return to the oven and bake for an additional 10 minutes, allowing the flavors to meld and the hazelnuts to toast slightly.

Transfer the hasselback butternut squash to a serving platter. Serve warm.