Carefully slice the spaghetti squash in half lengthwise using a sharp knife. Remove the seeds and any stringy bits inside.
Drizzle 2 tablespoons of olive oil on the cut sides of the squash and sprinkle with salt and pepper. Place the squash cut-side down on a baking sheet lined with parchment paper.
Roast the squash in the oven for about 40-45 minutes, or until the flesh is tender and can be easily pierced with a fork.
While the squash is roasting, heat the remaining 2 tablespoons of olive oil in a skillet over medium heat.
Add the minced garlic and crushed red pepper flakes (if using) to the skillet and sauté for about 2-3 minutes until fragrant but not browned.
Once the squash is done, remove it from the oven and let it cool for a few minutes. Using a fork, scrape the inside of the squash to create noodles.
Add the garlic oil mixture to the spaghetti squash noodles and toss to combine.
Stir in the grated Parmesan cheese, lemon zest, and additional salt and pepper to taste. Mix everything until well distributed.
Serve warm, garnished with fresh parsley on top.
Notes
For extra flavor, add more lemon zest or fresh herbs.