01 - Heat olive oil in a large saucepan over medium heat. Sauté onion until translucent, about 5 minutes. Add garlic and cook 1 minute until fragrant. Stir in crushed tomatoes, tomato paste, sugar, oregano, basil, red pepper flakes, salt, and black pepper. Simmer uncovered for 25 to 30 minutes, stirring occasionally until thickened. Adjust seasoning as needed.
02 - In a large skillet over medium-high heat, brown ground beef with salt, pepper, and nutmeg. Drain excess fat. Add 2 cups of prepared marinara sauce to beef and simmer for 5 minutes. Remove from heat and set aside.
03 - Combine ricotta cheese, egg, and grated Parmesan in a bowl until smooth and well incorporated.
04 - If using regular noodles, cook in a large pot of salted boiling water according to package directions. Drain and lay flat on a clean towel to prevent sticking.
05 - Preheat oven to 375°F. Spread 1 cup marinara sauce on bottom of a 9x13-inch baking dish. Layer 4 noodles over sauce. Spread half the ricotta mixture over noodles. Add half the beef mixture, then sprinkle with 2/3 cup shredded mozzarella. Repeat layering with noodles, remaining ricotta, beef mixture, and 2/3 cup mozzarella. Finish with final noodles, remaining marinara sauce, and remaining mozzarella.
06 - Cover with foil (tenting to prevent touching cheese) and bake 25 minutes. Remove foil and bake another 20 minutes until bubbly and golden. Let rest 15 minutes before slicing. Garnish with parsley if desired.