Place a large skillet over medium heat. Add the ground beef and onion and cook for 6-8 minutes, breaking up the meat as it cooks. Add the garlic, Italian seasoning, salt, pepper, and red pepper flakes. Stir to combine and cook for 1 minute.
Pour the marinara and water into the skillet. Stir everything together. Turn off the heat and let the meat sauce sit until ready to use.
In a medium mixing bowl, stir together the ricotta, 1 ½ cups mozzarella, ½ cup parmesan, egg, and parsley until evenly combined. Set aside.
In a 6-quart slow cooker, spread a layer of meat sauce over the bottom until it's just covered. Top with 3 lasagna noodles (you will need to break them to fit), then add a layer of 1/3 of the cheese mixture, and then a layer of 1/3 of the meat sauce. Repeat with the remaining ingredients, for a total of 3 layers. Sprinkle the rest of the mozzarella and parmesan over the top layer of meat sauce.
Secure the lid onto the crock pot and cook on LOW for 8 hours or on HIGH for 4 hours.
Remove the lid and turn off the slow cooker. Allow the lasagna to rest for 10 minutes before cutting into 6 portions and serving warm. Sprinkle with chopped parsley for garnish, if desired.