In a large skillet over medium high heat, brown ground beef and onion, stirring occasionally until browned. Add Italian seasoning and salt, mixing well. Add pasta sauce. Remove from heat, but leave in skillet.
In a medium bowl, combine ricotta cheese, parmesan cheese and 1 cup of the mozzarella cheese.
In slow cooker, add 2 cups of beef mixture into bottom of skillet and half of the noodles. Add spoonfuls of three cheese mixture on top of ziti, covering the noodles. Layer 2 more cups of beef, then remaining noodles, then cheese. Top with remaining beef mixture, ensuring cheese and pasta are covered.
Cover with lid, and cook on high for 4-5 hours or low for 6-7 hours, or until noodles are tender.
Ten minutes before serving, top with 1 cup mozzarella cheese, letting stand for ten minutes to melt.
If you end up with leftovers, be sure to freeze the extras for a great freezer meal!