Spaghetti squash is first roasted and the flesh is scraped out and combined with creamed spinach then layered with marinara sauce and topped with mozzarella for that stringy cheese-topped finish.
Scoop out the seeds of the spaghetti squash using a spoon and discard.
Preheat oven to 400ºF (200ºC). Rub the inside of each half with the olive oil. Bake until the spaghetti squash is tender when poked with a fork, about 45 minutes. Let cool for 6 minutes then use a fork to scrape flesh from top to bottom. Remove and set aside in a bowl.
For the marinara sauce:
Heat a medium saucepan over medium heat, add the olive oil and heat through. Add the onion and sauté until tender, stirring occasionally, about 6 minutes. Add the garlic and sauté for an additional minute.
Stir in the crushed tomatoes, parsley, oregano, basil and red pepper. Turn the heat down to medium-low and simmer the sauce for 20 minutes. Season the sauce to taste with salt and pepper.
For the creamed spinach:
Heat a large sauté pan over medium heat, add the olive oil and heat through. Add the garlic and sauté, stirring constantly, for 30 seconds.
Add the spinach, tossing to coat all the leaves in the oil. Sauté the spinach, stirring occasionally, until all the leaves have wilted, about 4 minutes. Remove to a large bowl and return the pan to the heat.
Add the butter and allow to melt. Add the flour, whisking to combine. Let the roux cook until a pale golden brown, about a minute. Add the milk, whisking to incorporate.
Bring the mixture to a simmer, then lower the heat to medium-low, whisking until the sauce thickens, about 2 minutes. Return the spinach to the pan and stir to combine. Season with salt then add the sauce over the bowl of spaghetti squash flesh, stirring to combine, and evenly coat the squash.
For assembly:
Heat oven to 425ºF (220ºC).
Arrange the spaghetti squash boats, cut side up on a baking sheet. Spread ¼ cup of marinara sauce in the bottom of each boat, then fill with the spinach and spaghetti squash mixture. Top each with an additional ¼ cup of the sauce then top with the mozzarella cheese.
Bake until the cheese has melted and begins to bubble, about 10 minutes. Remove and let cool then top with Parmesan cheese and arugula to garnish. Serve hot.