In a large Dutch oven, heat over medium heat, adding the ground beef and mild Italian sausage. Cook for 8-10 minutes until well browned, breaking it apart with a spatula. Once the meat is cooked through, add chopped onion and minced garlic, sautéing for another 3-4 minutes until the onion is translucent and fragrant.

Stir in the San Marzano tomatoes, crushed tomatoes, and tomato paste, mixing well. Sprinkle in the sugar, salt, pepper, and Italian seasoning, allowing the mixture to simmer on low heat for at least 30 minutes.

In a medium bowl, combine the ricotta cheese, egg, Parmesan cheese, and freshly chopped parsley. Season the mixture with salt and pepper to taste.

Bring a large pot of salted water to a rolling boil. Carefully add the lasagna noodles, cooking them for 6-8 minutes, allowing them to become al dente. Once done, drain the noodles and rinse them under cold water.

In a 9x13 inch lasagna pan, start with a thin layer of meat sauce at the bottom. Place a layer of noodles over the sauce, then spread half the ricotta mixture on top, followed by shredded mozzarella cheese.

Preheat your oven to 375°F (190°C). Cover your assembled lasagna with aluminum foil to keep it moist during baking. Bake for 30 minutes, then remove the foil and continue baking for an additional 15 minutes.

Once out of the oven, allow the lasagna to rest for 20-25 minutes before slicing. This crucial step helps the layers set.