Gently hand-mix all meatball ingredients in large bowl just until combined; do not overwork.
Roll about 40-50 balls, 3/4 inch across (smaller than golf ball); set aside on plate.
Heat oil in large pot over medium; sauté onion, carrots, and celery 6-7 minutes until softened.
Cook garlic 1 minute until fragrant, stirring constantly to prevent burning.
Add broth; boil over high.
Gently add meatballs individually; reduce to medium-low and simmer 10 minutes.
Add pasta; cook per package, usually 6-8 minutes, until al dente.
Cook spinach or escarole 2-3 minutes until wilted; season with salt and pepper.
Serve hot with generous Parmesan and crusty bread; flavor improves next day.