Bring a large pot of salted water to a boil. Add the 12 lasagna noodles and cook until al dente, about 8-9 minutes. Drain and rinse with cold water to stop the cooking process. Lay them flat on a clean kitchen towel to prevent sticking.
In a large skillet over medium heat, add the diced onion and sauté until translucent, about 3-4 minutes. Add the minced garlic and cook for another 30 seconds until fragrant. Add the ground beef and ground turkey, breaking them up with a spoon. Cook until browned and no longer pink. Drain any excess fat. Stir in the marinara sauce, dried oregano, salt, and pepper. Let simmer for 10 minutes so the flavors meld.
In a medium bowl, combine the ricotta cheese, egg, grated Parmesan, chopped fresh basil, salt, and pepper. Mix until smooth and well incorporated.
Preheat your oven to 375°F (190°C). In your 9x13-inch baking dish, spread a thin layer of the meat sauce on the bottom. Layer 4 lasagna noodles over the sauce, slightly overlapping. Spread half of the ricotta mixture over the noodles, followed by a third of the shredded mozzarella. Spoon over a third of the meat sauce. Repeat the layers: noodles, ricotta, mozzarella, meat sauce. Finish with a final layer of noodles, meat sauce, and the remaining mozzarella cheese sprinkled on top.
Cover the casserole tightly with foil and bake for 30 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown. Let the casserole rest for 10 minutes before slicing. This rest time ensures the layers set beautifully for neat serving.