01 - Heat olive oil in a large saucepan over medium heat. Sauté the chopped onion until translucent, about 5 minutes. Add minced garlic and cook for 1 minute until fragrant. Stir in crushed tomatoes, tomato paste, sugar, oregano, basil, red pepper flakes, salt, and black pepper. Simmer uncovered for 25–30 minutes, stirring occasionally, until thickened. Adjust seasoning as necessary.
02 - In a large skillet over medium-high heat, brown ground beef seasoned with salt, black pepper, and nutmeg. Drain excess fat. Add 2 cups of prepared marinara sauce to the beef and simmer for 5 minutes. Remove from heat and set aside.
03 - Combine ricotta cheese, large egg, and grated Parmesan cheese in a bowl. Mix thoroughly until well blended.
04 - If using regular noodles, cook them in a large pot of salted boiling water according to package instructions. Drain and spread noodles flat on a clean towel to prevent sticking.
05 - Preheat oven to 375°F. Spread 1 cup marinara sauce evenly on the bottom of a 9x13-inch baking dish. Layer 4 noodles over the sauce. Spread half of the ricotta mixture over the noodles, followed by half of the beef filling. Sprinkle with 2/3 cup shredded mozzarella. Repeat layering with noodles, remaining ricotta mixture, beef filling, and 2/3 cup mozzarella. Finish with a final layer of noodles, remaining marinara sauce, and remaining mozzarella cheese.
06 - Cover the dish with tented aluminum foil to prevent cheese from sticking. Bake for 25 minutes. Remove foil and bake an additional 20 minutes until bubbly and golden brown. Allow to rest for 15 minutes before slicing. Garnish with chopped fresh parsley, if desired.