Step 1: Prepare the meat sauce. Heat olive oil in a large skillet over medium heat. Add ground beef/sausage and onion; cook until meat is browned and onion is tender. Drain any excess fat. Add minced garlic and cook for 1 minute until fragrant.
Step 2: Stir in crushed tomatoes, tomato sauce, Italian seasoning, oregano, and red pepper flakes (if using). Bring to a simmer, then reduce heat, cover, and let it cook for at least 20-30 minutes, stirring occasionally, to allow flavors to meld. Season with salt and pepper to taste.
Step 3: While the sauce simmers, cook lasagna noodles according to package directions until al dente. Drain, rinse with cold water to prevent sticking, and lay flat on parchment paper or foil.
Step 4: Prepare the ricotta mixture. In a medium bowl, combine ricotta cheese, egg, 1/2 cup Parmesan cheese, and chopped parsley. Mix well and season with a pinch of salt and pepper.
Step 5: Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish. Spread a thin layer of meat sauce on the bottom of the prepared dish.
Step 6: Layer the lasagna. Arrange 3-4 lasagna noodles over the sauce. Spread half of the ricotta mixture evenly over the noodles. Top with 1/3 of the remaining meat sauce, then sprinkle with 1/3 of the mozzarella cheese and a layer of pepperoni slices.
Step 7: Repeat the layering: noodles, remaining ricotta, 1/3 meat sauce, 1/3 mozzarella, and pepperoni. Add a final layer of noodles, the last of the meat sauce, and the remaining mozzarella cheese. Top with a generous sprinkle of Parmesan cheese and more pepperoni.
Step 8: Cover the baking dish tightly with aluminum foil. Bake for 30 minutes. Remove the foil and bake for an additional 20-25 minutes, or until the cheese is bubbly and golden brown. If desired, broil for the last few minutes for extra browning, watching carefully to prevent burning.
Step 9: Let the lasagna rest for 10-15 minutes before slicing and serving. This allows the layers to set and prevents it from falling apart.