Heat the olive oil in a large pot or Dutch oven over medium heat. Add the onion and cook until softened, about 5 minutes. Add the garlic and cook for 1 minute more, until fragrant.

Add the ground beef and Italian sausage to the pot. Cook, breaking up the meat with a spoon, until browned. Drain off any excess grease.

Stir in the crushed tomatoes, tomato sauce, tomato paste, oregano, basil, salt, pepper, and parsley. Bring to a simmer, then reduce heat and cook for at least 30 minutes, or up to 1 hour, stirring occasionally. The longer it simmers, the richer the flavor will be.

In a large bowl, combine the ricotta cheese, Parmesan cheese, parsley, egg, salt, and nutmeg. Mix well until everything is evenly combined.

While the sauce is simmering, cook the lasagna noodles according to package directions until al dente. Drain and rinse with cold water to prevent sticking. If using no-boil noodles, set aside.

Preheat oven to 375°F (190°C). Spread a thin layer of meat sauce in the bottom of a 9x13 inch baking dish.

Layer noodles over the sauce, overlapping as needed to cover the bottom of the dish. If using no-boil noodles, ensure the sauce generously coats the noodles to allow them to soften during baking.

Spread half of the ricotta cheese mixture over the noodles. Sprinkle with 1 cup of mozzarella cheese.

Spoon half of the remaining meat sauce over the mozzarella cheese.

Repeat layers: noodles, remaining ricotta cheese mixture, 1 cup mozzarella cheese, remaining meat sauce.

Top with the remaining 2 cups of mozzarella cheese.

Cover the baking dish with aluminum foil and bake for 30 minutes. Remove the foil and bake for an additional 15-20 minutes, or until the cheese is melted and bubbly and the lasagna is heated through.

Let the lasagna rest for at least 15 minutes before slicing and serving. This allows the layers to set and prevents it from falling apart.