Place the ground turkey, breadcrumbs, egg, garlic powder, oregano, salt, and black pepper in a mixing bowl. Mix with your hands until the ingredients are evenly combined.
Shape the mixture into 16 meatballs roughly the size of golf balls and set them on a plate.
Heat a large skillet over medium heat. Add the meatballs and cook, turning every few minutes, until they are browned on all sides.
Pour the marinara sauce into the skillet. Lower the heat, cover, and let the meatballs simmer in the sauce for 15 minutes.
While the meatballs simmer, bring the water to a boil in a medium saucepan. Slowly whisk in the instant polenta and cook according to package directions, usually 3 to 5 minutes.
Remove the polenta from heat and stir in the butter until it melts and the texture turns smooth.
Spoon the polenta into bowls. Top each serving with meatballs and sauce, then finish with Parmesan cheese and parsley.