Preheat oven to 350 degrees. Combine all ingredients in a large bowl, and mix well (preferably with your hands).
Divide the mixture into 4 fairly equal portions. Each portion should yield 12 to 15 equal-sized meatballs. Roll them as firmly as possible.
Place the meatballs on a lightly greased baking pan, and place in the preheated oven for 5 minutes or until lightly browned. Rotate meatballs to brown evenly, and remove from oven to cool
Soup
Heat olive oil in a stock pot over medium heat.
Add onion, celery, garlic, carrot, red pepper flakes and basil leaves, and cook until onions are translucent, about 5 minutes.
Add broth and diced tomato, and bring to a boil. Reduce heat to low, and simmer for 5 minutes.
Add meatballs to soup, and simmer for 5 minutes more.
Add spinach leaves, and simmer for 2 minutes. Season to taste with salt and pepper.
To serve, ladle soup into bowls and top with cheese.