Tender turkey meatballs add plenty of savory flavor to this hearty, vegetable-packed soup. Poaching the meatballs directly in the broth is one trick to making it taste so rich; another is to add a Parmesan rind while the broth simmers.
Gently combine turkey, breadcrumbs, oregano, and 1 tablespoon of the oil in a bowl. Shape mixture into 28 small, 1/2-inch balls.
Heat remaining 2 tablespoons oil in a large Dutch oven over medium-high. Add onion, celery, and carrots, and cook, stirring occasionally, until vegetables are softened, 8 to 10 minutes. Stir in stock, Parmesan rind, and salt; bring to a boil. Add couscous; cook, stirring occasionally, 7 minutes. Add meatballs, zucchini, and squash; cook, stirring occasionally, until meatballs are cooked through and vegetables are crisp-tender, about 3 minutes. Stir in spinach and parsley. Remove and discard Parmesan rind.
Ladle soup into bowls; sprinkle each serving with 1 tablespoon grated Parmesan.