In a large frying pan, brown the sausage over med-high heat. When almost cooked through, add onions, garlic, salt & pepper and 1 jar of marinara sauce and stir to combine.
Spray slow cooker with cooking spray. If using a cooking bag, spray the inside with cooking spray.
Spread 1/3 of the meat sauce on the bottom of the slow cooker. Top with 1/3 of the uncooked lasagna noodles. Break some pieces to make them fit. Top with 1/3 of ricotta and 1/2 cup of mozzarella cheese. Repeat these layers 2 more times.
Pour the second jar of marinara sauce over the lasagna and top with remaining mozzarella cheese.
Pour the water in on the sides.
Cook on low for 6 hours.