Cook ground beef and onion in a large skillet over medium-high heat, stirring occasionally, until beef is no longer pink. Stir in seasonings to evenly coat meat; stir in pasta sauce. Set aside.
Mix 1 cup mozzarella, the ricotta and parmesan in a medium bowl. Spoon 2 cups of meat sauce into the slow cooker; top with 2 cups ziti. Drop half of the cheese mixture, by rounded tablespoons, over ziti; carefully spread over ziti using back of spoon. Layer with 2 cups meat sauce, the remaining ziti and remaining cheese. Add remaining meat sauce, completely covering all the ziti and cheese. Place lid on slow cooker.
Cook on low for 6 to 7 hours or high for 4 to 5 hours or until noodles are tender.
Carefully remove lid to allow steam to escape. Sprinkle food with the remaining 1 cup mozzarella; let stand 10 minutes until melted. Serve directly from slow cooker.