Preheat oven to 400°F, cut spaghetti squash in half lengthwise, scoop out seeds, drizzle with olive oil, and season with salt and pepper.
Place squash cut-side down on a baking sheet and roast for 40-45 minutes until tender.
Use a fork to scrape the squash into spaghetti-like strands, then press strands with paper towels to remove excess moisture.
Mix the squash strands with garlic, Italian seasoning, and 1 cup marinara sauce, then divide back into the squash shells or transfer to a baking dish.
Top with shredded chicken, remaining marinara sauce, mozzarella, and Parmesan cheese, then bake at 375°F for 20-25 minutes until cheese is melted and bubbly.
Garnish with fresh basil before serving.