Step 1: Prepare the Meat Sauce: Heat olive oil in a large pot or Dutch oven over medium heat. Add ground beef and Italian sausage, breaking it up with a spoon. Cook until browned, draining any excess fat.
Step 2: Add chopped onion and minced garlic to the pot. Sauté until the onion is softened, about 5-7 minutes.
Step 3: Stir in crushed tomatoes, tomato paste, water (or broth), oregano, basil, sugar, salt, and pepper. Bring to a simmer, then reduce heat to low, cover, and let it simmer for at least 30 minutes, or up to 1 hour, stirring occasionally. This allows flavors to meld.
Step 4: Prepare Ricotta Mixture: In a large bowl, combine ricotta cheese, egg, Parmesan cheese, fresh parsley, nutmeg (if using), and milk. Season with a pinch of salt and pepper. Mix until well combined and creamy.
Step 5: Cook Lasagna Noodles (if not oven-ready): If using traditional noodles, cook according to package directions until al dente. Drain and rinse with cold water to prevent sticking.
Step 6: Assemble the Lasagna: Preheat oven to 375°F (190°C). Spread a thin layer of meat sauce on the bottom of a 9x13 inch baking dish.
Step 7: Lay 3-4 lasagna noodles over the sauce (trimming if necessary to fit). Spread half of the ricotta mixture evenly over the noodles.
Step 8: Top the ricotta with about 1.5 cups of meat sauce, then sprinkle with 1/3 of the mozzarella cheese.
Step 9: Repeat the layers: noodles, remaining ricotta mixture, meat sauce, and another 1/3 of the mozzarella cheese.
Step 10: Add a final layer of noodles, top with the remaining meat sauce, and sprinkle generously with the last of the mozzarella cheese and a bit more Parmesan.
Step 11: Bake: Cover the baking dish tightly with aluminum foil (you can spray the underside with cooking spray to prevent cheese from sticking). Bake for 45 minutes.
Step 12: Remove the foil and bake for another 15-20 minutes, or until the cheese is bubbly and lightly browned.
Step 13: Rest and Serve: Let the lasagna rest for 15-20 minutes before slicing and serving. This is crucial for the layers to set properly.