In a medium bowl, mix turkey, ground bread crumbs, egg, half of the onions, 1/2 teaspoon salt and black pepper. Mix well until it becomes compact.
Make small balls then push a few pieces of the mozzarella at the center of each meatball. Roll the meatballs on a plate with flour then set aside. Over medium heat, in a saucepan with olive oil, brown the outside of the meatballs. When they are browned, set them aside in a bowl. Discard the oil. Note that the meatballs do not need to be cooked through all the way as they will cook in the sauce and slow cooker.
Over medium heat, still in the same saucepan, pour some new oil then sautè the remaining onion. Put back the meatballs then pour in the broth. Cook until the sauce thickens.
In the slow cooker, pour the meatballs with the sauce, canned tomatoes, oregano, sage, the remaining salt and a dash of pepper. Set the slow cooker to low for 6 hours or high for 4 hours.