Cook sausage and onion in a skillet over medium heat, stirring occasionally, until sausage is no longer pink; drain well.
Stir in tomato sauce, basil, oregano and salt.
In a bowl, combine 2 cups of the shredded mozzarella cheese, the ricotta cheese and parmaesan cheese. Refrigerate the remaining mozzarella cheese.
Spoon 1/4 of the sausage mixture into the crockpot; top with 4 lasagna noodles broken into pieces to fit.
Top with half the cheese mixture. Repeat until all gone. Finish with the sausage mixture on top of noodles.
Cover and cook on low heat for 6 to 8 hours, or until noodles are tender.
Sprinkle top of lasagna with remaining 1 cup of mozzarella cheese. Cover and let stand 10 minutes, until cheese melts.