Step 1: Preheat and Prepare the Squash Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper for easy cleanup. Carefully slice each spaghetti squash in half lengthwise. Use a sturdy spoon to scoop out all the seeds and stringy pulp from the center. Discard the seeds or save them for roasting later.
Step 2: Season and Roast the Squash Halves Drizzle the cut sides of the squash with 1 tablespoon of olive oil. Sprinkle the insides evenly with 1 teaspoon of salt and 1/2 teaspoon of black pepper. Place the squash halves cut-side down on the prepared baking sheet. Roast for 40-50 minutes, until the outer skin is tender and can be easily pierced with a fork, and the inside strands pull away easily.
Step 3: Sauté the Aromatics and Meat While the squash roasts, prepare the filling. In a large skillet over medium-high heat, add the remaining 1 tablespoon of olive oil. Add the ground Italian sausage, breaking it up with a wooden spoon. Cook for 5-7 minutes, until browned. Add the diced onion and cook for another 3-4 minutes, until softened. Stir in the minced garlic and cook for 1 minute more, until fragrant.
Step 4: Simmer the Filling Sauce Pour the marinara sauce into the skillet with the meat. Add the dried oregano and red pepper flakes (if using). Stir everything together, then reduce the heat to low. Let the sauce simmer gently while the squash finishes roasting, about 10 minutes, to allow the flavors to meld.
Step 5: Shred the Squash Strands Once the squash is cool enough to handle (about 5 minutes), flip them over. Using a fork, gently scrape the flesh of the squash from edge to edge. You’ll see it separate into beautiful, spaghetti-like strands. Leave the strands in the shell to create a natural “boat” for the filling.
Step 6: Stuff and Assemble the Boats Divide the meat sauce evenly among the four squash halves, spooning it directly over the shredded strands. Top each boat with a generous sprinkle of mozzarella cheese and Parmesan cheese.
Step 7: Final Bake to Melt and Bubble Return the assembled squash boats to the baking sheet. Bake for 10-15 minutes, or until the cheese is fully melted, bubbly, and starting to turn golden brown in spots.
Step 8: Garnish and Serve Remove the baking sheet from the oven. Let the boats rest for 2-3 minutes. Garnish with fresh, torn basil leaves. Serve immediately while hot and cheesy.