Step 1: In a large skillet over medium-high heat, brown the ground beef and Italian sausage with the diced onion until the meat is no longer pink. Add the minced garlic and cook for 1 additional minute. Drain the excess fat thoroughly.
Step 2: Stir the marinara sauce into the meat mixture. Season with salt and pepper. Remove from heat and set aside.
Step 3: In a medium mixing bowl, combine the ricotta cheese, egg, and fresh parsley. Stir until well blended.
Step 4: Spread about 1 cup of the meat sauce onto the bottom of a 6-quart slow cooker. Place a layer of uncooked lasagna noodles over the sauce, breaking them to fit the shape of the pot.
Step 5: Spread 1/3 of the ricotta mixture over the noodles, then top with 1/3 of the remaining meat sauce and 1 cup of mozzarella cheese. Repeat the layers (noodles, ricotta, sauce, mozzarella) twice more.
Step 6: Finish with a final layer of noodles, the remaining meat sauce, the rest of the mozzarella, and the grated Parmesan cheese.
Step 7: Cover and cook on LOW for 3.5 to 4.5 hours. The lasagna is done when the noodles are tender and the cheese is bubbly. Let it stand for 15 minutes before serving to help the layers set.