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. Remove from heat and set aside.
03 - In a bowl, combine ricotta cheese, egg, and grated Parmesan cheese until smooth.
04 - If using regular noodles, cook them in a large pot of salted boiling water according to package instructions. Drain and lay flat on a clean towel to prevent sticking.
05 - Preheat oven to 375 degrees Fahrenheit. Spread 1 cup marinara sauce on the bottom of a 9x13-inch baking dish. Layer 4 noodles over the sauce. Spread half of the ricotta mixture evenly over noodles. Top with half the beef mixture, then sprinkle with 2/3 cup shredded mozzarella. Repeat layers: noodles, remaining ricotta mixture, remaining beef, 2/3 cup mozzarella. Finish with a final layer of noodles, remaining marinara sauce, and remaining mozzarella cheese.
06 - Cover the dish with foil tented so it does not touch the cheese. 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 chopped parsley if desired.