Preheat the oven to 350 degrees and line a baking sheet with foil.
In a large bowl, mix together the ingredients for the meatballs. Form balls that are about 1” in width and place them on the baking sheet. Bake for 15 - 20 minutes, until cooked through.
Add olive oil to a large pot over medium heat. Add the onion, carrots, and celery. Cook and stir for 6 - 8 minutes, until tender.
Pour in the broth into the pot and bring it to a boil. Add the pasta and cook for as long as directed on the package (usually 9 - 10 minutes). Pasta should be cooked al dente (still slightly firm in the middle). Taste and season the soup with additional salt and pepper if desired.
Turn the heat to low and add the spinach and cooked meatballs. Simmer for 3 - 5 minutes, until the spinach is wilted.
Remove the pot from the heat and stir in the lemon juice. Serve warm, optionally topped with parmesan and chopped parsley.