Preheat your oven to 400°F. Slice the spaghetti squash in half lengthwise and scoop out the seeds. Drizzle the cut sides with olive oil, brush to coat evenly, and season with salt and pepper. Place the squash cut-side down on a baking sheet and roast for about 30 minutes, or until the flesh is tender and easily shreds with a fork.
While the squash roasts, prepare the meat sauce. Heat a large skillet over medium heat and cook the chicken sausage, breaking it up as it browns, for about 5-7 minutes. Add the onion and garlic and sauté for another 2-3 minutes, or until the onion is softened.
Stir in the marinara sauce, Italian seasoning, and red pepper flakes. Mix well, cover, and let the sauce simmer until the spaghetti squash is finished cooking.
In a small bowl, combine the ricotta cheese, parmesan cheese, and Italian seasoning until well mixed.
Once the squash is done, use a fork to gently loosen the strands while keeping them inside the shells. Layer each squash half with some of the meat sauce, a spoonful of the ricotta mixture, and another layer of meat sauce. Top with shredded mozzarella.
Return the squash to the oven for about 5 minutes, or until the cheese is melted and bubbly. For a golden, lightly crisp top, broil for the last 1-2 minutes.
Garnish with fresh basil and serve warm.