Prepare the spaghetti squash: if the stem is long, carefully slice just below the flesh of the skin to remove it. Cut the spaghetti squash in half crosswise, and scoop out the seeds.
Place the frozen meatballs into the bottom of the slow cooker and pour the marinara sauce over them. Add the minced onion, garlic, basil, oregano, salt, and pepper.
Place the prepared squash, cut-side down, on top of the meatballs and sauce.
Cover and cook on high heat for 3 – 4 hours, or low heat for 6 – 7 hours (or until the squash is soft).
Remove the squash using tongs, and use a fork to scrape the strands of “spaghetti” away from the skin of the squash.
Serve the spaghetti squash strands topped with sauce and meatballs. Garnish with parmesan cheese, if desired.