01 - Heat olive oil in a large saucepan over medium heat. Add onion and sauté until translucent, about 5 minutes. Stir in garlic and cook for 1 minute until fragrant. Add crushed tomatoes, tomato paste, sugar, oregano, basil, red pepper flakes, salt, and 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 the ground beef with salt, pepper, and nutmeg. Drain excess fat. Add 2 cups of the prepared marinara sauce to the beef and simmer for 5 minutes. Set aside.
03 - In a bowl, mix ricotta cheese, egg, and grated Parmesan until well combined.
04 - If using regular noodles, cook them in a large pot of salted boiling water according to package directions. Drain and lay flat on a clean towel.
05 - Preheat oven to 375°F (190°C). Spread 1 cup of marinara sauce on the bottom of a 9x13-inch baking dish. Layer 4 noodles over the sauce. Spread half of the ricotta mixture over noodles. Top with half the beef mixture, then sprinkle with 2/3 cup mozzarella cheese. Repeat layers: noodles, remaining ricotta, remaining beef, 2/3 cup mozzarella. Finish with final layer of noodles, remaining marinara sauce, and remaining mozzarella cheese.
06 - Cover with foil, tenting to avoid contact with cheese, and bake for 25 minutes. Remove foil and bake an additional 20 minutes until bubbly and golden. Let rest for 15 minutes before slicing. Garnish with parsley if desired.