Step 1: In a large skillet, brown the ground beef (or sausage) and chopped onion over medium-high heat. Drain any excess grease. Add minced garlic and cook for another minute until fragrant.
Step 2: Stir in the marinara sauce, crushed tomatoes, Italian seasoning, salt, and pepper. Bring to a simmer, then reduce heat and let it cook gently for 10-15 minutes to allow flavors to meld. This is your delicious meat sauce for the crockpot lasagna.
Step 3: In a medium bowl, combine the ricotta cheese, egg, chopped fresh parsley, and 1/2 cup Parmesan cheese. Mix well until smooth. This is your creamy cheese mixture.
Step 4: Lightly grease the inside of a 6-quart (or larger) oval slow cooker. Spread about 1 cup of meat sauce on the bottom of the slow cooker. This prevents sticking and helps the first layer of noodles cook evenly for your crockpot lasagna.
Step 5: Place 2-3 no-boil lasagna noodles over the sauce, breaking them if necessary to fit the shape of your slow cooker. Don't worry if they overlap slightly. These noodles will cook perfectly in the slow cooker for your crockpot lasagna.
Step 6: Spread about half of the ricotta cheese mixture evenly over the noodles. Top with about 1.5 cups of shredded mozzarella cheese.
Step 7: Add another 1.5 cups of meat sauce over the mozzarella, spreading it gently.
Step 8: Repeat the layers: noodles, remaining ricotta mixture, 1.5 cups mozzarella, and the rest of the meat sauce. Finish with a final layer of noodles.
Step 9: Top the last layer of noodles with the remaining mozzarella cheese and a generous sprinkle of Parmesan cheese. This will create a beautiful cheesy crust on your crockpot lasagna.
Step 10: Cover the slow cooker and cook on HIGH for 3-4 hours or on LOW for 5-6 hours, or until the noodles are tender and the cheese is bubbly and melted. Cooking times can vary based on your slow cooker, so check for doneness. The final product should be a perfectly cooked crockpot lasagna.
Step 11: Once cooked, turn off the slow cooker and let the crockpot lasagna rest, covered, for 15-20 minutes before serving. This allows the layers to set, making it easier to slice and serve.