01 - In a large skillet over medium heat, brown ground beef or Italian sausage until cooked through. Drain off excess fat. Add diced onion and minced garlic, and sauté for 2 to 3 minutes until softened.
02 - Incorporate crushed tomatoes, tomato paste, tomato sauce, dried basil, dried oregano, salt, black pepper, red pepper flakes if desired, and sugar into the skillet. Simmer for 8 to 10 minutes to blend flavors.
03 - In a mixing bowl, combine ricotta cheese, egg, and parsley until smooth and fully incorporated.
04 - Lightly grease the inside of the slow cooker. Spread a thin layer of prepared meat sauce on the bottom.
05 - Arrange 3 to 4 no-boil lasagna noodles in the slow cooker, breaking as needed to fit. Top with half the ricotta mixture, one-third of the mozzarella, and one-third of the meat sauce. Repeat layers: noodles, remaining ricotta filling, another third mozzarella, and another third sauce. Finish with a final layer of noodles, remaining sauce, remaining mozzarella, and Parmesan cheese.
06 - Cover and cook on LOW setting for 4 to 5 hours until noodles are tender and cheese is melted.
07 - Turn off heat and let stand for 15 to 20 minutes before slicing and serving for clean portions.