Defrost the bag of Italian meatballs overnight. Add the meatballs to the slow cooker. Add the sliced onions as well. Pour in the crushed tomatoes and sprinkle on all of the seasonings. Mix until the ingredients are evenly distributed. Pour in the beef stock. Cook on high for 2 hours, or low for 4 hours. Serve on prepared spaghetti noodles, or zucchini "zoodles," with some parmesan cheese.