Step 1: In a large pot or Dutch oven, heat olive oil over medium-high heat. Add ground beef and Italian sausage, breaking it up with a spoon as it cooks. Cook until no pink remains, then drain off any excess fat.
Step 2: Add chopped onion and minced garlic to the pot. Sauté until the onion is softened and translucent, about 5-7 minutes.
Step 3: Stir in crushed tomatoes, tomato paste, water or beef broth, dried oregano, dried basil, and red pepper flakes (if using). Season generously with salt and pepper. Bring the sauce to a simmer, then reduce heat to low, cover, and let it gently simmer for at least 1 hour, or up to 2-3 hours, stirring occasionally.
Step 4: While the sauce simmers, prepare the ricotta mixture. In a medium bowl, combine ricotta cheese, egg, 1/2 cup Parmesan cheese, chopped fresh parsley, salt, and pepper. Mix until well combined.
Step 5: Bring a large pot of salted water to a rolling boil. Add lasagna noodles and cook according to package directions until al dente. Drain the noodles and rinse them with cold water to prevent sticking. Lay them flat on parchment paper or a clean kitchen towel.
Step 6: Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish. Spread a thin layer of meat sauce (about 1 cup) on the bottom of the prepared dish.
Step 7: Lay 3-4 lasagna noodles over the sauce, slightly overlapping. You may need to break some to fit the dish.
Step 8: Evenly spread half of the ricotta cheese mixture over the noodles.
Step 9: Top the ricotta with a generous layer of meat sauce (about 1.5 cups), then sprinkle with about a third of the shredded mozzarella cheese.
Step 10: Repeat the noodle, ricotta, sauce, and mozzarella layers: noodles, remaining ricotta, sauce, and mozzarella.
Step 11: For the final layer, place noodles, top with the remaining meat sauce, and sprinkle generously with the remaining mozzarella and a final dusting of Parmesan cheese.
Step 12: Cover the baking dish tightly with aluminum foil (you can lightly spray the underside of the foil with cooking spray to prevent cheese from sticking).
Step 13: Bake for 25 minutes.
Step 14: Remove the foil and bake for another 20-25 minutes, or until the cheese is bubbly and golden brown. If desired, broil for 1-2 minutes for a darker, bubbly top (watch carefully!).
Step 15: Let the lasagna rest for 10-15 minutes before slicing and serving. This allows the layers to set and prevents it from falling apart.