Preheat the oven to 375°F (190°C).

Cook the lasagna noodles according to package directions until al dente. Drain and lay them flat to avoid sticking.

In a large skillet over medium heat, sauté the chopped onion until translucent, about 4 minutes. Add the garlic and cook for 30 seconds until fragrant.

Add the ground beef and Italian sausage. Break up the meat and brown until no pink remains. Drain excess fat and return the skillet to low heat. Stir in 1 cup of the marinara sauce to keep the meat moist.

In a medium bowl, combine ricotta, the egg, Italian seasoning, and a pinch of salt and pepper. Mix until smooth.

Spread a thin layer of marinara on the bottom of a 9x13-inch baking dish to prevent sticking.

Arrange 3 lasagna noodles over the sauce. Spoon half the ricotta mixture over the noodles and spread gently. Top with half the meat, 1 cup marinara, and 1 cup shredded mozzarella.

Repeat the layers: 3 noodles, remaining ricotta, remaining meat, another cup of marinara, and 1 cup mozzarella.

Finish with the final 3 noodles, the remaining marinara, the remaining mozzarella, and sprinkle the grated Parmesan on top.

Cover the dish tightly with foil and bake for 25 minutes. Remove the foil and bake for another 25 minutes, until the cheese is bubbly and golden.

Let the lasagna rest for 10 minutes before slicing to help it set. Serve warm.