Heat the oven to 400°F (200°C) and line a baking sheet with parchment if desired. Carefully cut the spaghetti squash in half lengthwise, then scoop out and discard the seeds and stringy pulp.

Brush the cut surfaces with about 1 tablespoon of the olive oil. Season the squash evenly with the kosher salt and freshly ground black pepper.

Place the squash halves cut-side down on the prepared baking sheet. Roast for 40–50 minutes, beginning to check around 40 minutes, until the shell is easy to pierce and the flesh separates into tender strands.

Let the roasted squash cool for several minutes so it is easier to handle. Use a fork to scrape the flesh lengthwise into spaghetti-like strands.

Toss the warm strands with the butter, remaining olive oil if needed, Parmesan, and chopped parsley. Taste and add more salt and pepper as desired, then serve warm.