In a medium bowl, gently combine ground beef, ground turkey, 1/4 cup Parmesan, 1 egg, breadcrumbs, 2 tablespoons minced parsley, salt, pepper, and garlic powder; portion mixture with a teaspoon or small scoop and roll into 3/4- to 1-inch meatballs.

Heat 1 tablespoon olive oil in a 12-inch skillet over medium-high heat; sear meatballs in a single layer 30–45 seconds per side to form a thin crust without cooking through, then transfer to a plate.

In a 6-quart pot, heat 1 tablespoon olive oil over medium; sauté carrot, celery, and onion until softened, about 6–8 minutes, then add garlic and cook 30 seconds.

Pour in 8 cups low-sodium chicken broth and bring to a gentle simmer; return seared meatballs to the pot and simmer gently 12–14 minutes until meatball centers reach 165°F.

If cooking orzo separately, boil 1 cup orzo in salted water 6–7 minutes until slightly al dente, drain, and hold; alternatively, stir dry orzo into the simmering soup and cook 8–9 minutes.

Stir in 3 cups packed spinach in the last 2 minutes until wilted.

Whisk remaining 1 egg with 2 tablespoons chopped parsley and 1 tablespoon Parmesan, temper with 1/3 cup hot broth, then slowly stream back into the pot while stirring to create an egg-parsley ribbon.

Remove from heat, adjust salt and pepper, add a squeeze of lemon if desired, and serve with extra Parmesan.