In a large skillet over medium-high heat, brown the ground beef and diced onion until the meat is no longer pink (about 5-7 minutes). Add minced garlic and cook for another 30 seconds until fragrant. Drain excess fat if necessary.

Stir in Italian seasoning, salt, pepper, marinara sauce, tomato sauce, and tomato paste. Add water to thin the sauce slightly. Bring to a simmer for 5 minutes to allow flavors to blend, then remove from heat.

In a medium bowl, combine ricotta cheese, egg, parsley, 1 cup of mozzarella, and 1/4 cup of Parmesan cheese. Mix well until smooth and evenly combined.

Lightly coat your slow cooker with cooking spray. Spread about 1 cup of the meat sauce on the bottom of the slow cooker.

Place a layer of uncooked lasagna noodles on top of the sauce, breaking them as needed to fit and cover the surface.

Spread about 1/3 of the ricotta cheese mixture over the noodles, then sprinkle with 1/2 cup mozzarella.

Add about 1 1/2 cups of meat sauce over the cheese. Repeat the layers (noodles, ricotta mixture, mozzarella, meat sauce) two more times.

Top with a final layer of noodles, the remaining sauce, and the remaining mozzarella and Parmesan cheeses.

Cover and cook on LOW for 4-5 hours or on HIGH for 2-3 hours, until noodles are tender.

Turn off the heat and let the lasagna rest for 15-20 minutes before serving. This helps it set and makes it easier to slice.