Preheat the oven to 425°F(220°C). Line a large baking sheet with parchment paper.
Rub the flesh of the butternut squash with olive oil. Season well with salt and pepper.
Set the butternut squash halves, cut side down, on the sheet.
Bake for 40 minutes, until fork tender. Remove from the oven and let cool slightly.
Scoop out the flesh, leaving about 1/4” of squash around the skin (don’t break the skin or it will collapse), and place the flesh in a large bowl.
Add the sour cream, 1/2 cup of Parmesan, salt, and pepper to the bowl. Mix it all up until it’s well combined. Spoon it back into the butternut squash shells. Place them face up on the baking sheet. Sprinkle half of the remaining Parmesan over the top. Bake for another 15 minutes. Then turn on the broil and broil the tops until they’re golden brown, about 5 minutes.