In a non-stick skillet over medium-high heat, cook the ground beef with the diced onion and minced garlic until the beef is fully browned. Season the beef with the salt, pepper, and Italian seasoning and mix to combine. Drain any excess fat and set aside.
Coat the inside of your slow cooker with non-stick cooking spray. Spread 1 cup of marinara sauce evenly on the bottom.
Place 3-4 lasagna noodles on the bottom of the slow cooker, breaking them as needed to fit.
Spoon 1/3 of the ricotta cheese over the noodles and spread it evenly.
Sprinkle 1/4 of the shredded mozzarella cheese and 1 tablespoon of grated Parmesan cheese over the ricotta layer.
Evenly layer 1/3 of the cooked ground beef mixture on top of the cheeses.
Pour another cup of marinara sauce over the beef layer. Repeat the layering steps (noodles, ricotta, mozzarella, Parmesan, beef, marinara) two more times.
Add a final layer of lasagna noodles, followed by the remaining marinara sauce and the rest of the shredded mozzarella cheese.
Cover the slow cooker and cook on HIGH for 3 hours or on LOW for 5-6 hours, until the noodles are tender.
Let the lasagna cool for 10-15 minutes before slicing and serving. Sprinkle with any remaining Parmesan cheese and garnish with fresh basil or parsley if desired. Enjoy!