01 - In a large skillet over medium heat, crumble and cook ground beef or sausage until evenly browned. Drain excess fat. Add diced onion and minced garlic, cooking for 2 to 3 minutes until fragrant and softened.
02 - Stir in crushed tomatoes, tomato paste, tomato sauce, dried basil, dried oregano, salt, black pepper, optional red pepper flakes, and sugar. Simmer gently for 8 to 10 minutes, stirring occasionally.
03 - In a mixing bowl, combine ricotta cheese, egg, and parsley. Mix thoroughly until the mixture is smooth and fully blended.
04 - Lightly grease the inside of the slow cooker insert with nonstick spray or a small amount of oil.
05 - Spread a thin layer of prepared meat sauce on the bottom. Arrange 3 to 4 no-boil lasagna noodles over the sauce, breaking noodles as needed to fit. Add half of the ricotta mixture, one-third of the mozzarella, and one-third of the remaining meat sauce. Repeat the layering process with noodles, the rest of the ricotta, another third of mozzarella, and another third of sauce. Finish with a final layer of noodles, the remaining sauce, the last third of mozzarella, and parmesan cheese.
06 - Cover the slow cooker with its lid and cook on LOW setting for 4 to 5 hours, until noodles are tender and the cheese is completely melted.
07 - Turn off the heat and allow the lasagna to rest, uncovered, for 15 to 20 minutes before slicing and serving. This helps the layers set for easier serving.