In a large Dutch oven, brown ground beef and mild Italian sausage over medium heat until fully cooked, about 8-10 minutes. Add chopped onion and minced garlic, sautéing until softened, around 3-4 minutes. Stir in San Marzano tomatoes, crushed tomatoes, tomato paste, sugar, and season generously with salt, pepper, and Italian seasoning. Let this simmer for at least 30 minutes.

In a mixing bowl, combine ricotta, beaten egg, and freshly grated Parmesan cheese. Add chopped parsley, plus salt and pepper to taste. Mix thoroughly until well blended.

Bring a pot of salted water to boil and cook lasagna noodles until just shy of al dente, about 6-8 minutes. Drain and run under cold water to stop the cooking process.

In a lasagna pan, spread a thin layer of meat sauce on the bottom. Lay down noodles over the sauce, spread half of the ricotta mixture, and sprinkle shredded mozzarella. Repeat layering, finishing with noodles topped with remaining meat sauce and extra mozzarella.

Preheat oven to 375°F (190°C). Cover pan tightly with aluminum foil and bake for 30 minutes. Remove foil and continue baking for 15 minutes until cheese is golden.

Let lasagna rest for 20-25 minutes before serving.