In a large skillet, cook the ground beef over medium heat, breaking it into small chunks as it cooks. Drain any excess fat.
Add the chopped onion to the skillet and sauté until softened, about 5 minutes.
Pour in the spaghetti sauce and water, stirring well, and bring to a simmer. Let simmer for about 5 minutes, then remove from heat.
In a medium bowl, mix together the ricotta cheese, 1 cup of mozzarella cheese, parmesan cheese, egg, and dried parsley until well combined.
Spread 1 cup of the tomato sauce mixture in the bottom of the slow cooker to prevent sticking.
Add 3 lasagna noodles, breaking them into pieces if necessary to cover the bottom loosely.
Spread half of the ricotta cheese mixture evenly over the noodles, followed by another cup of the tomato sauce mixture.
Add another layer of noodles, followed by the remaining ricotta mixture and another cup of tomato sauce.
Top with the last layer of noodles, pouring the remaining tomato sauce over the top.
Cover the slow cooker and cook on low for 4 hours, or until the noodles are soft.
Sprinkle the remaining mozzarella cheese on top about 10 minutes before serving. Cover and let it cook for an additional 10 to 15 minutes, until melted.
Turn off the heat and uncover the slow cooker. Allow it to sit for about 15 minutes before serving.