Preheat your oven to 400°F (200°C). Line a shallow baking pan with a rack and set aside.
In a large bowl, whisk together the eggs, spinach, breadcrumbs, Parmesan cheese, dried onion, garlic, salt, and pepper. Add the ground chicken and mix well until fully combined. Shape the mixture into 1 1/2-inch meatballs and place them on the prepared rack.
Bake the meatballs uncovered for 20–25 minutes until they’re no longer pink in the center and reach an internal temperature of 165°F.
Meanwhile, prepare the spaghetti squash. Cut it lengthwise and scoop out the seeds, then place each half cut-side down on a microwave-safe plate. Microwave on high for 15–18 minutes until tender.
As the squash cooks, heat olive oil in a nonstick skillet over medium heat. Saute the mushrooms until they soften and release their juices, about 5 minutes. Stir in the diced tomatoes, tomato sauce, parsley, garlic, oregano, and basil. Bring to a boil, then lower the heat and simmer for 8–10 minutes, allowing the flavors to meld.
Once the sauce thickens slightly, add the baked meatballs to the pan and heat through for 2–3 minutes.
Use a fork to scrape the spaghetti squash into strands. Serve the meatballs and sauce over the squash, garnished with additional Parmesan cheese if desired.