Preheat the oven. Set your oven to 400°F (200°C).
Line a baking sheet with parchment paper for easy cleanup.
Prep the squash. Carefully cut the spaghetti squash in half lengthwise. Scoop out the seeds with a spoon. Drizzle the cut sides with olive oil and season with salt and pepper.
Roast cut-side down. Place the squash halves cut-side down on the baking sheet.
Roast for 35–45 minutes, depending on size, until the flesh is tender and strands pull away easily with a fork.
Warm the marinara. While the squash roasts, heat your marinara in a saucepan over medium heat. If you like, sauté the minced garlic in a little olive oil first, then add the sauce and Italian seasoning. Simmer for 10–15 minutes to deepen the flavor.
Shred the squash. When the squash is done, let it cool for a few minutes.
Use a fork to scrape the flesh into strands, leaving some structure in the shell if you want to serve it “boat-style.” Taste and adjust salt and pepper.
Combine and heat through. Add the strands to the saucepan of marinara and toss gently, or spoon warmed sauce over the squash in its shell. Heat until everything is nicely coated and hot.
Finish and serve. Top with chopped basil or parsley. Add grated Parmesan and a pinch of red pepper flakes if you like.
Serve right away while it’s steamy and fragrant.