In a skillet over medium heat, cook the beef and onions for about 5 minutes, or until beef is no longer pink. Drain, if needed.
Season with salt, pepper, garlic, and Italian seasoning.
Stir in the marinara sauce and add 1/2 cup water to the jar. Shake around and pour into skillet. Simmer for 5-10 minutes.
Meanwhile, in a bowl, combine the ricotta, egg, 1/2 cup parmesan cheese, basil, and some salt and pepper.
Evenly spread 1 cup of the marinara into the bottom of the slow cooker. Top with a layer of noodles, breaking them up as needed to fit. Gently spread 1/4-1/3 of the ricotta mixture on top. Add a layer of mozzarella cheese.
Repeat the layers two or three more times, depending on the size of your crockpot (I had 4 layers total). End with the mozzarella cheese layer. Sprinkle with 1/4 cup of parmesan cheese.
Cover and cook on low for about 4 hours, or until noodles are tender.
Allow to rest for 10 minutes before cutting. top with fresh basil if desired.
Notes
This recipe can absolutely be made vegetarian. Just use your favorite marinara sauce in place of the meat sauce!