Cook the Beef and Veggies: Sauté ground beef, chopped onions, and mushrooms until browned and tender. Drain the fat.
Make the Sauce: Add marinara, oregano, salt, pepper, and red pepper flakes. Simmer and set aside.
Boil the Noodles: Cook lasagna noodles until al dente. Rinse under cold water and drain well.
Mix Ricotta Filling: Combine ricotta, eggs, Parmesan, salt, and pepper until smooth.
Preheat Oven: Set the oven to 375°F (190°C).
Assemble the Layers: In a greased baking dish, layer sauce, noodles, ricotta, mozzarella, pepperoni—repeat until all ingredients are used.
Bake: Cover with foil and bake for 35 minutes until hot and bubbly.
Cool and Serve: Rest for 10–15 minutes before slicing. Garnish with parsley.