Preheat oven to 450 degrees and line a baking sheet with aluminum foil.
In a small bowl, soak the bread in the milk. In a large bowl, mix the chicken or turkey, Parmesan, 1/2 teaspoon salt, pepper, egg, and the bread soaked in milk. Mix well until it becomes compact.
Get about a tablespoon of the meat then roll in between your palms to make the meatballs—we made them using a 1 inch scoop. Place them on the baking sheet. Cook for 6 minutes.
For the sauce, heat the olive oil on medium-low. Add the onions and garlic and saute until onions are translucent. Add the onions, garlic, and all additional ingredients to a bowl and whisk until smooth.
Add enough meatballs to cover the bottom of the slow cooker, then pour on half of the sauce. Add the remaining meatballs and pour the rest of the sauce. Cover and cook on low 2-4 hours, or until meatballs are cooked through.