Step 1: In a large skillet over medium heat, brown the ground beef or sausage until fully cooked. Drain the excess fat and stir in the marinara sauce.
Step 2: In a medium bowl, combine the ricotta cheese, beaten egg, Parmesan cheese, and dried oregano. Mix well until smooth.
Step 3: Spread about 1 cup of the meat sauce onto the bottom of a 6-quart slow cooker. Splash the 1/2 cup of water or broth around the edges.
Step 4: Place a layer of dry lasagna noodles over the sauce, breaking them into pieces as needed to cover the area.
Step 5: Spread 1/3 of the ricotta mixture over the noodles, followed by a layer of mozzarella cheese and more meat sauce.
Step 6: Repeat the layers (noodles, ricotta, cheese, sauce) twice more, finishing with a final generous layer of mozzarella cheese on top.
Step 7: Cover and cook on LOW for 4 to 5 hours or on HIGH for 2.5 to 3 hours. Ensure the noodles are tender by piercing with a fork.
Step 8: Turn off the slow cooker and let the lasagna rest for 15-20 minutes before slicing to allow the layers to set.