Brown the ground beef in a skillet over medium heat. Add the diced onion and minced garlic, and cook until the onion softens.
Stir the marinara sauce into the beef mixture and remove the skillet from heat.
In a mixing bowl, combine the ricotta cheese, egg, Italian seasoning, salt, and black pepper.
Spread a thin layer of the meat sauce in the bottom of the slow cooker. Add a layer of broken noodles, followed by half the ricotta mixture and one cup of mozzarella.
Repeat the layers with more sauce, noodles, the remaining ricotta mixture, and the rest of the mozzarella.
Finish with a final layer of sauce and sprinkle the Parmesan cheese on top.
Cover and cook on low for 4 hours until the noodles are tender.