Preheat the Oven: Preheat your oven to 375°F (190°C).
Cook the Meat: In a large skillet, heat olive oil over medium heat. Add the ground beef, Italian sausage, and onion. Cook until browned, about 8-10 minutes. Drain excess fat.
Add Garlic and Sauce: Add the minced garlic to the skillet and cook for 1-2 minutes. Then stir in the marinara sauce, Italian seasoning, salt, and pepper. Let it simmer for 10 minutes.
Prepare Cheese Mixture: In a medium mixing bowl, combine ricotta cheese, 1 cup of mozzarella cheese, Parmesan cheese, and egg. Mix well.
Layer the Casserole: In a 9x13-inch baking dish, spread a thin layer of meat sauce on the bottom. Add a layer of lasagna noodles, then a layer of the cheese mixture. Repeat the layers until all ingredients are used, ending with a layer of meat sauce on top.
Top with Cheese: Sprinkle the remaining 1 cup of mozzarella cheese over the top.
Bake: Cover the casserole with aluminium foil and bake for 25 minutes. Remove the foil and bake for an additional 15 minutes until the cheese is bubbly and golden brown.
Garnish and Serve: Let the casserole cool for 5-10 minutes before serving. Garnish with fresh parsley if desired.