Cook sausage and onion in 10-inch skillet over medium heat 6 to 8 minutes, stirring occasionally, until sausage is no longer pink; drain. Stir in tomato sauce, basil and salt.
Mix 1 cup of the mozzarella cheese and the ricotta and Parmesan cheeses. (Refrigerate remaining mozzarella cheese while lasagna cooks.)
Spoon one-fourth of the sausage mixture into 6-quart slow cooker; top with 5 noodles, broken into pieces to fit. Spread with half of the cheese mixture and one-fourth of the sausage mixture. Top with 5 noodles, remaining cheese mixture and one-fourth of the sausage mixture. Top with remaining 5 noodles and remaining sausage mixture.
Cover and cook on Low heat setting 4 to 6 hours or until noodles are tender.
Sprinkle top of lasagna with remaining 1 cup mozzarella cheese. Cover and let stand about 10 minutes or until cheese is melted. Cut into pieces.