In a large skillet, cook ground beef or sausage over medium heat until browned, then drain excess fat
Add chopped onion and garlic, cooking for another 2 minutes until fragrant
Stir in pasta sauce, diced tomatoes, Italian seasoning, salt, and black pepper, then remove from heat
In a separate bowl, mix ricotta cheese, egg, and half of the mozzarella cheese
Lightly grease the crockpot, then spread a thin layer of the meat sauce on the bottom
Layer 3-4 uncooked lasagna noodles over the sauce, breaking them as needed to fit
Spread a layer of the ricotta cheese mixture over the noodles, then add another layer of meat sauce
Repeat layers until all ingredients are used, finishing with meat sauce on top
Pour ½ cup of water around the edges to help the noodles cook
Cover and cook on low for 4-5 hours or high for 2-3 hours until noodles are tender
Sprinkle the remaining mozzarella and Parmesan cheese on top, cover, and let melt for 10-15 minutes
Let the lasagna sit for 10 minutes before serving