01 - Heat olive oil in a large saucepan over medium heat. Sauté onion until translucent, about 5 minutes. Add 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 needed.
02 - In a large skillet over medium-high heat, brown ground beef with salt, black pepper, and nutmeg. Drain excess fat. Add 2 cups of prepared marinara sauce and simmer for 5 minutes. Remove from heat and set aside.
03 - Combine ricotta cheese, egg, and grated Parmesan in a bowl. Mix until smooth and well incorporated.
04 - If using regular noodles, cook 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°F. Spread 1 cup marinara sauce on the bottom of a 9x13-inch baking dish. Layer 4 lasagna noodles over sauce. Spread half the ricotta mixture over noodles, then half the beef filling. Sprinkle with 2/3 cup mozzarella cheese. Repeat layering: noodles, remaining ricotta, remaining beef, 2/3 cup mozzarella. Finish with final noodles, remaining marinara sauce, and remaining mozzarella.
06 - Cover dish with foil tented to avoid touching 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 parsley if desired.