Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper or a silicone baking mat.
In a large mixing bowl, combine the ground turkey, spinach, breadcrumbs, egg, garlic, onion, oregano, salt, and pepper. Mix gently until just combined—don’t overmix, or the meatballs will become tough.
Use a cookie scoop or your hands to form the mixture into meatballs, about 1.5 inches in diameter. Place them on the prepared baking sheet, spacing them about an inch apart.
Bake in the preheated oven for 18-20 minutes, or until the meatballs are golden brown and cooked through (the internal temperature should reach 165°F).
While the meatballs are baking, make the creamy sauce. In a large skillet, melt the butter over medium heat. Add the garlic and cook for 1-2 minutes, until fragrant.
Stir in the milk, Parmesan cheese, salt, pepper, and red pepper flakes (if using). Cook for 5-7 minutes, stirring occasionally, until the sauce thickens slightly.
Once the meatballs are done, add them to the skillet with the creamy sauce. Toss gently to coat the meatballs in the sauce.
Serve hot, garnished with extra Parmesan cheese if desired.