**Cook the lasagna noodles**
Bring a large pot of salted water to a boil. Cook the lasagna noodles according to the package instructions until al dente. Drain and rinse with cold water to prevent sticking. Lay the noodles flat on a clean towel or parchment paper.
**Prepare the meat sauce**
In a large skillet or saucepan, cook the ground beef, Italian sausage, and diced onion over medium heat until browned and fully cooked. Drain any excess grease. Add the minced garlic and cook for 1 minute until fragrant. Stir in the marinara sauce, crushed tomatoes, basil, oregano, salt, and pepper. Simmer the sauce for 15–20 minutes, stirring occasionally.
**Mix the cheese filling**
In a mixing bowl, combine the ricotta cheese, egg, Parmesan cheese, and parsley. Stir until smooth and set aside.
**Assemble the lasagna**
Preheat your oven to 375°F (190°C). Spread a thin layer of meat sauce on the bottom of a 9x13-inch baking dish. Layer 3–4 lasagna noodles over the sauce, slightly overlapping them. Spread 1/3 of the ricotta mixture over the noodles, followed by 1/3 of the meat sauce and 1 cup of shredded mozzarella. Repeat the layers two more times, ending with a final layer of meat sauce and mozzarella. Sprinkle Parmesan cheese on top.
**Bake the lasagna**
Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 15–20 minutes, or until the cheese is bubbly and golden brown.
**Cool and serve**
Let the lasagna rest for 10–15 minutes before slicing to allow the layers to set. Garnish with fresh parsley if desired and serve warm.