Preheat oven to 400°F (200°C).
Slice the spaghetti squash in half lengthwise and scoop out the seeds.
Drizzle the cut sides of the squash with olive oil and season with salt and pepper.
Place squash cut-side down on a baking sheet and roast for 40 to 45 minutes until tender.
Remove squash from oven and use a fork to scrape the flesh into spaghetti-like strands, keeping them inside the shell.
In a bowl, combine shredded chicken and marinara sauce.
Distribute the chicken and sauce mixture evenly into the squash shells, mixing slightly with the squash strands.
Top each half with shredded mozzarella, parmesan cheese, and breadcrumbs.
Return to the oven or place under a broiler for 5 to 10 minutes until the cheese is melted and golden brown.
Garnish with fresh parsley before serving.