In a large skillet or Dutch oven, brown the ground beef and Italian sausage over medium-high heat, breaking the meat apart as it cooks. Drain excess fat, then add the diced onion and cook for 2-3 minutes until softened. Stir in the garlic and cook for another minute.

Add the marinara sauce, basil, oregano, red pepper flakes (if using), salt, and pepper. Reduce heat and let the sauce simmer gently for 15 minutes, allowing the flavors to develop.

In a large bowl, mix ricotta, cream cheese, and sour cream until smooth. Stir in 1 cup of mozzarella, Parmesan cheese, the egg, and parsley. Mix well until fully combined and creamy.

Bring a large pot of salted water to a boil and cook the lasagna noodles according to package instructions until al dente. Drain and rinse with cold water to prevent sticking.

Preheat the oven to 375°F (190°C). Spread a thin layer of meat sauce in a greased 9x13-inch baking dish. Add a layer of noodles, then spread half of the cheese mixture evenly over them. Top with meat sauce and a layer of shredded mozzarella.

Repeat the layers, finishing with noodles topped with the remaining meat sauce. Sprinkle generously with mozzarella and Parmesan cheese.

Cover with foil and bake for 25 minutes. Remove the foil and bake an additional 20-25 minutes, until the top is golden and bubbly. Let the lasagna rest for 15 minutes before slicing and serving.