Heat olive oil in a large skillet or Dutch oven over medium heat. Add the chopped onion and cook until softened, then stir in the garlic and cook for 1 more minute.

Add the ground beef and Italian sausage. Cook until browned, breaking the meat into small pieces as it cooks. Drain excess grease if needed.

Stir in the tomato paste, crushed tomatoes, diced tomatoes, Italian seasoning, red pepper flakes, salt, and black pepper. Simmer uncovered for 30 to 40 minutes, stirring occasionally, until the sauce thickens.

In a medium bowl, mix the ricotta cheese, egg, and half of the Parmesan cheese until smooth.

Preheat the oven to 375°F.

Spread a thin layer of meat sauce in the bottom of a 9×13-inch baking dish.

Add a layer of lasagna noodles, then spread some ricotta mixture over the noodles, add mozzarella cheese, and spoon over more meat sauce.

Repeat the layers until all ingredients are used, finishing with meat sauce on top and the remaining mozzarella and Parmesan cheese.

Cover the dish with foil and bake for 25 minutes.

Remove the foil and bake for another 20 to 25 minutes, until the cheese is melted, bubbling, and lightly golden.

Let the lasagna rest for 15 minutes before slicing and serving.