Preheat your oven to 400°F (200°C).
Cut the spaghetti squash in half lengthwise and scoop out the seeds.
Drizzle the cut sides with olive oil and season with salt and pepper.
Place the squash halves cut side down on a baking sheet and roast for 40-45 minutes, until tender.
While the squash is roasting, heat a tablespoon of olive oil in a pan over medium heat. Add minced garlic and cook until fragrant.
Add chopped spinach to the pan and cook until wilted. Mix in marinara sauce and remove from heat.
Once the squash is done, use a fork to scrape the flesh into spaghetti-like strands.
Mix the squash strands with the marinara and spinach mixture. Fill the squash shells with the mixture and top with shredded mozzarella cheese.
Return the stuffed squash to the oven and bake for another 10 minutes, or until the cheese is melted and bubbly.