01 - In a large skillet set over medium heat, brown the ground beef or Italian sausage, breaking it up into small pieces. Once browned, drain off any excess fat. Add the diced onion and minced garlic, sautéing for 2 to 3 minutes until aromatic and softened.
02 - Add the crushed tomatoes, tomato paste, tomato sauce, dried basil, dried oregano, salt, black pepper, red pepper flakes (if using), and sugar to the skillet. Stir well and simmer for 8 to 10 minutes to combine flavors.
03 - In a mixing bowl, combine ricotta cheese, egg, and parsley. Blend until mixture is smooth and fully integrated.
04 - Lightly grease the inside of the slow cooker. Spread a small amount of meat sauce onto the bottom. Layer 3 to 4 oven-ready lasagna noodles, breaking them as needed to fit. Top with half of the ricotta mixture, then add one-third of the mozzarella cheese and one-third of the meat sauce.
05 - Repeat the layers: noodles, the remaining ricotta mixture, another third of the mozzarella, and another third of the meat sauce. Finish with a final layer of noodles, the rest of the meat sauce, remaining mozzarella, and sprinkle with grated Parmesan cheese.
06 - Cover and cook on low for 4 to 5 hours, or until the noodles are tender and the cheeses are thoroughly melted.
07 - Turn off the slow cooker and let the lasagna rest for 15 to 20 minutes. Slice and serve warm.