01 - Heat a drizzle of olive oil in a large skillet over medium heat. Add ground beef and onion; cook until meat is browned and onion is softened, approximately 6 to 7 minutes. Stir in garlic and cook for 1 minute. Drain any excess fat.
02 - Stir marinara sauce, dried basil, dried oregano, salt, and black pepper into the skillet. Simmer for 2 to 3 minutes. Remove from heat.
03 - In a medium mixing bowl, combine ricotta cheese, egg, chopped parsley, and half of the grated Parmesan cheese. Mix thoroughly until smooth.
04 - Spray the inside of a 6-quart slow cooker with nonstick cooking spray. Spread approximately 1 cup of meat sauce onto the bottom.
05 - Break lasagna noodles as necessary to fit a single layer over the sauce. Spread one third of the ricotta mixture over the noodles, followed by one third of the mozzarella cheese and a layer of meat sauce. Repeat layering twice more, finishing with sauce and mozzarella cheese on top. Sprinkle with remaining Parmesan cheese.
06 - Cover and cook on LOW heat for 4 hours, or until noodles are tender and cheese is melted and bubbling.
07 - Allow lasagna to rest in the slow cooker, uncovered, for 20 to 30 minutes to set before slicing and serving.