How to Make It
Stir together breadcrumbs and milk in a large bowl; soak 5 minutes. Add egg, onion, Parmesan, vinegar, salt, and pepper; stir to combine. Add turkey, stirring gently until just combined. Using your hands, gently shape mixture into 16 (1 1/2-inch) balls.
Heat oil in a large nonstick skillet over medium-high. Add meatballs; cook, turning often, until browned on all sides, 8 to 10 minutes. Transfer to a plate.
Reduce heat under skillet to medium-low; add marinara and spinach. Increase heat to medium, and bring to a simmer. Simmer until spinach just begins to wilt, 2 to 3 minutes. Return meatballs to sauce in skillet; sprinkle with mozzarella. Cover and cook until meatballs are cooked through, 4 to 6 minutes. Sprinkle with basil; serve immediately.