In a large mixing bowl, use your hands to mix together ½ pound ground beef, ½ pound ground pork, ½ cup Parmesan cheese, ¼ cup Italian seasoned breadcrumbs, ¼ cup yellow onion, 2 large eggs, 2 tablespoons parsley, 1 clove garlic, ¼ teaspoon dried oregano, ¼ teaspoon salt, and ¼ teaspoon black pepper until evenly combined. Do not overmix.
Use a cookie scoop to portion the meat mixture into 1.5 tablespoon-sized scoops. Then, use your hands to roll the scoops into meatballs and place them on a plate.
Place a Dutch oven over medium heat and add 1 tablespoon olive oil once the pot is hot. Brown the meatballs on all sides in batches. You do not need to cook them all the way through. Once browned, place them back on the plate. This will take about 15 minutes. Don't wipe out the pot.
Add 1 tablespoon olive oil, 1 ¼ cups carrots, 1 ¼ cups yellow onion, and ¾ cup celery to the pot. Cook, stirring occasionally, until the veggies have softened, about 10 minutes. Stir in 4 cloves garlic and cook until fragrant, about 30 seconds.
Add 9 cups low-sodium chicken broth and use a wooden spoon to scrape off any browned bits from the bottom of the pan. Bring to a boil.
Add the browned meatballs and 1 cup orzo pasta and stir to combine. Cover and cook for 10 minutes, stirring occasionally to keep anything from sticking to the bottom. The meatballs are done when they reach an internal temperature of 160°F.
Stir in 6 ounces spinach until wilted. Portion into bowls and serve immediately with more Parmesan cheese on top.