Spray the slow cooker insert with nonstick cooking spray.
Make the meat sauce: Add olive oil to a large skillet or pot. Add in sausage and ground beef and cook until golden brown and no longer pink. Add in the onion and garlic and saute for an additional 2 minutes or so to develop some color. Stir in tomato sauce, water and Italian seasoning. Season with salt and pepper to taste.
Assemble: Layer 1/4 of the meat sauce onto the bottom of the slow cooker. Top with 3 lasagna noodles breaking them up so they fit. It is okay if they overlap. Spread 1/3 of the ricotta on top of the noodles then sprinkle with 1/4 cup parmesan and 1/2 cup mozzarella cheese. Repeat with two more layers or until you run out of ingredients. (Make sure to reserve 1/2 cup of mozzarella for topping after cooking). The last layer should be tomato sauce.
Cook: For 4 hours on low heat or until noodles are tender. Sprinkle the remaining 1/2 cup of mozzarella on top and cover to melt the cheese. This should take only a couple of minutes.
Serve: I let the lasagna rest for 15-30 minutes to make slicing easier. Serve with fresh basil for garnish, if desired.