Preheat the oven to 400°F (200°C). Mix the meatball ingredients, form into 1-tablespoon-sized balls, and bake for 20-25 minutes.
In a large pot, heat the oil and sauté the onion, carrot, and celery for 7-10 minutes. Add the garlic, fennel, and red pepper flakes, and cook for 1 minute.
Pour in the broth, Italian seasoning, pasta, and cooked meatballs. Bring to a boil, then simmer for 8 minutes until the pasta is tender.
Stir in the Parmesan cheese and spinach, and allow the cheese to melt and spinach to wilt.
Add lemon juice, parsley, and season with salt and pepper to taste. Serve hot and enjoy