In a large skillet, cook ground beef, breaking it into small chunks. Drain the fat. Add chopped onion to the skillet and cook until softened.
Add spaghetti sauce and water to the skillet and bring it to a simmer. Simmer 5 minutes, then remove from heat.
In a medium bowl, mix ricotta, mozzarella cheese, parmesan cheese, egg, and parsley.
Spread 1 cup of the tomato sauce in the bottom of the slow cooker. Add 3 lasagna noodles, breaking them into pieces as needed to cover the bottom loosely. Spread half of the ricotta mixture evenly over the noodles, followed by 1 cup of the tomato sauce.
Add another layer of noodles, followed by the other half of the ricotta mixture. Add the final layer of noodles, then spread the remaining tomato sauce over the top.
Cook the lasagna on low for 4 hours, or until the noodles are soft. Sprinkle with the remaining mozzarella cheese and cover. Cook 10-15 minutes, or until the mozzarella is melted. Turn off the heat, uncover, and let sit 15 minutes before serving.