In a large mixing bowl, combine the ground beef, breadcrumbs, egg, grated Parmesan, 2 cloves minced garlic, 1 tbsp chopped parsley, 1 tsp salt, and 1/2 tsp black pepper. Mix gently with your hands until just combined, being careful not to overmix.

Roll the mixture into approximately 1-inch meatballs. You should get about 24-28 meatballs.

Heat 1 tbsp olive oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Add the meatballs in batches, ensuring not to overcrowd the pot. Brown the meatballs on all sides until golden, about 3-5 minutes per batch. They do not need to be cooked through. Remove the browned meatballs with a slotted spoon and set them aside on a plate.

Reduce the heat to medium. If needed, add another 1 tbsp of olive oil to the pot. Add the chopped onion, diced carrots, and diced celery. Cook, stirring occasionally, until the vegetables begin to soften, about 5-7 minutes.

Add the remaining 3 cloves of minced garlic to the pot and cook for 1 minute more until fragrant.

Stir in the tomato paste and cook for another minute, stirring to coat the vegetables.

Pour in the chicken broth, diced tomatoes (undrained), Italian seasoning, 1/2 tsp salt, 1/4 tsp black pepper, and the bay leaf. Bring the mixture to a gentle simmer, scraping up any browned bits from the bottom of the pot.

Return the browned meatballs to the pot. Add the orzo pasta.

Bring the soup back to a simmer, then reduce the heat to medium-low. Cover the pot and cook for 15-20 minutes, or until the orzo is tender and the meatballs are cooked through. Stir occasionally to prevent the orzo from sticking to the bottom of the pot.

Remove and discard the bay leaf.

Stir in the fresh spinach until it has wilted, which will take about 2-3 minutes.

Taste the soup and adjust seasoning (salt and pepper) if necessary.

Ladle the Hearty Meatball Orzo Soup into bowls and serve hot, garnished with freshly grated Parmesan cheese and a sprinkle of chopped fresh parsley.