Step 1: In a large skillet over medium-high heat, brown the ground beef (or sausage) and chopped onion. Drain any excess grease. Add minced garlic and cook for another minute until fragrant.
Step 2: Stir in the crushed tomatoes, tomato sauce, tomato paste, water/broth, Italian seasoning, salt, and pepper. Bring to a simmer, then reduce heat and let it cook for 10-15 minutes, allowing flavors to meld. This is your meat sauce.
Step 3: In a separate medium bowl, combine the ricotta cheese, 1/2 cup Parmesan cheese, egg (if using), 2 tablespoons chopped parsley, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mix well.
Step 4: Lightly grease the inside of a 6-quart (or larger) oval slow cooker. Spread about 1 cup of the meat sauce evenly on the bottom of the slow cooker.
Step 5: Arrange a layer of no-boil lasagna noodles over the sauce. Break noodles as needed to fit the shape of your crockpot, ensuring they cover as much surface as possible without overlapping too much.
Step 6: Spoon half of the ricotta mixture over the noodles, spreading it gently. Top with about 1.5 cups of shredded mozzarella cheese.
Step 7: Add another layer of meat sauce (about 1.5 cups). Repeat with another layer of noodles, the remaining ricotta mixture, and 1.5 cups of mozzarella cheese.
Step 8: Place a final layer of noodles, then top with the remaining meat sauce and the remaining 1 cup of mozzarella cheese. Sprinkle with a little extra Parmesan if desired.
Step 9: Cover the slow cooker and cook on LOW for 3.5 to 4 hours, or until the noodles are tender and the cheese is bubbly and melted. Cooking time may vary slightly depending on your slow cooker.
Step 10: Once cooked, turn off the slow cooker and let the lasagna rest, covered, for 15-20 minutes. This allows the layers to set and makes for easier serving. Garnish with fresh parsley before slicing and serving.