Step 1: In a large skillet over medium-high heat, brown the ground beef, Italian sausage, and minced garlic until fully cooked. Drain the excess fat thoroughly.
Step 2: Stir the marinara sauce and dried oregano into the meat mixture. Let it simmer for 2-3 minutes then remove from heat.
Step 3: In a medium bowl, combine the ricotta cheese, egg, Parmesan cheese, and nutmeg. Mix until smooth and well combined.
Step 4: Spread about 1 cup of the meat sauce onto the bottom of a 6-quart slow cooker.
Step 5: Place a layer of dry lasagna noodles over the sauce, breaking them to fit the shape of the pot.
Step 6: Spread one-third of the ricotta mixture over the noodles, followed by 1 cup of shredded mozzarella and 1.5 cups of meat sauce.
Step 7: Repeat the layers (noodles, ricotta, mozzarella, sauce) two more times. Finish with a final layer of noodles topped with the remaining sauce and mozzarella cheese.
Step 8: Cover and cook on LOW for 4 to 5 hours. Once finished, turn off the heat and let the lasagna rest for 15-20 minutes with the lid slightly ajar to set before serving.