Preheat the oven to 400 degrees.
In a large mixing bowl, combine the turkey, onion, garlic, egg, salt, breadcrumbs, and Parmesan. Mix until well combined, and then form the mixture into golfball-sized meatballs.
Grease a baking sheet with the olive oil, then arrange the meatballs in an even layer. Bake for 15 minutes.
After 15 minutes of baking, turn the meatballs using tongs. Pour the passata over the top and return to the oven for an additional 10 minutes.
Meanwhile, begin making the polenta. Bring the water and milk to a boil in a large stockpot. Add the salt and reduce the heat to medium.
Sprinkle the polenta into the pot very slowly, whisking constantly from the moment you begin adding the polenta until the mixture begins to thicken, about 10 minutes.
Remove the meatballs from the oven and keep warm. Meanwhile, continue cooking the polenta an additional 10 minutes, or until it begins to pull away from the sides of the pot.
Remove the polenta from the heat and add the Italian seasoning and butter. Serve the polenta topped with the meatballs and sauce.