Step 1: Heat olive oil in a large pot or Dutch oven over medium heat. Add finely diced onion and cook until softened, about 5-7 minutes. Add minced garlic and cook for another minute until fragrant.
Step 2: Add the ground beef and Italian sausage to the pot. Break up the meat with a spoon and cook until browned, about 8-10 minutes. Drain any excess fat.
Step 3: Stir in the crushed tomatoes, tomato paste, dried oregano, dried basil, red pepper flakes (if using), and a bay leaf. Season with salt and black pepper. Bring to a simmer, then reduce heat to low, cover, and cook for 1.5-2 hours, stirring occasionally. Remove bay leaf before using.
Step 4: In a medium bowl, combine the drained whole-milk ricotta cheese, 1/2 cup grated Parmesan cheese, egg, chopped fresh parsley, salt, and black pepper. Mix until well combined.
Step 5: Cook lasagna noodles according to package directions until al dente, if not using oven-ready. Drain and lay flat to prevent sticking.
Step 6: Preheat oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish. Spread a thin layer of meat sauce on the bottom of the dish.
Step 7: Place a layer of lasagna noodles over the sauce. Spread half of the ricotta mixture evenly over the noodles. Top with about 1.5 cups of meat sauce, then sprinkle generously with shredded mozzarella cheese.
Step 8: Repeat the noodle, ricotta, sauce, and mozzarella layers. For the final layer, place noodles, top with the remaining meat sauce, and then a generous amount of mozzarella and Parmesan cheese.
Step 9: Cover the baking dish tightly with aluminum foil. Bake for 25 minutes. Remove the foil and bake for an additional 20-25 minutes, or until the cheese is bubbly and golden brown. Let rest for 15-20 minutes before slicing and serving.