Prepare the meat sauce:
In a skillet over medium heat, cook the ground beef or sausage until browned. Add the chopped onion and garlic, and cook until the onion is soft, about 3-4 minutes. Drain excess fat, then stir in the marinara sauce and water. Season with salt and pepper if desired. Set aside.
Prepare the cheese mixture:
In a mixing bowl, combine ricotta cheese, 1 cup of the shredded mozzarella, Parmesan cheese, egg, and Italian seasoning. Mix until smooth.
Assemble the lasagna in the crockpot:
Spread a thin layer of the meat sauce on the bottom of the crockpot.
Break the lasagna noodles as needed to fit and create a layer over the sauce.
Spread a layer of the ricotta mixture over the noodles, followed by a layer of meat sauce.
Repeat layers (noodles, ricotta mixture, and meat sauce) until all ingredients are used, finishing with a layer of meat sauce on top. Sprinkle with the remaining 1 cup of mozzarella cheese.
Cook:
Cover and cook on low for 4-5 hours or on high for 2-3 hours, until the noodles are tender and the cheese is melted.
Serve:
Allow the lasagna to rest for 10-15 minutes before slicing. Serve warm.