Cook ground beef, chopped onion, and minced garlic in a large skillet over medium heat until beef is browned. Drain excess fat.
Stir in pasta sauce and tomato sauce; simmer for about 5 minutes to blend flavors.
In a separate bowl, mix ricotta cheese, 1 1/2 cups shredded mozzarella, grated Parmesan, and egg until creamy.
Spread a thin layer of meat sauce on the bottom of the slow cooker.
Arrange a layer of oven-ready noodles over the sauce, breaking as needed to fit.
Spread a portion of the cheese mixture over the noodles.
Repeat layering meat sauce, noodles, and cheese mixture until all ingredients are used, finishing with a layer of meat sauce on top.
Pour 2 cups of water around the edges of the slow cooker to help noodles cook evenly.
Cover and cook on low for 4 to 5 hours until noodles are tender and fully cooked.
About 10 minutes before serving, sprinkle remaining shredded mozzarella on top, cover again, and let it melt for a bubbly, golden finish.