Preheat Oven: Preheat oven to 350°F (175°C).
Prepare Noodles: Bring a large pot of salted water to a boil. Cook lasagna noodles for 8 minutes until just tender. Drain and lay the noodles out on a greased cookie sheet to cool and dry.
Mix Ricotta: In a medium bowl, combine ricotta cheese, egg, Parmesan cheese, 1 teaspoon of Italian seasoning, and salt and pepper to taste. Set aside.
Cook sausage: Heat large skillet to medium-high. Add Italian sausage to skillet. Cook over medium-high until browned, crumbled, and set aside. Reserve about ½ cup for topping later.
Cook Veggies: Using the same skillet with the onions and mushrooms, until tender. Add garlic and cook an additional 1-2 minutes.
Spread a thin layer of marinara sauce (about ½ cup) in the bottom of a 9×13-inch baking dish.
Layer with 4 lasagna noodles. Half of the ricotta mixture should be spread over these noodles, followed by sausage, veggies, and 1 cup of mozzarella cheese. Spoon ⅓ of the remaining marinara sauce over the cheese.
Add another layer of noodles: 4, remaining ricotta mixture, pepperoni set aside for topping, 1 cup of mozzarella cheese, and half of the remaining marinara sauce.
Cover the remaining noodles, marinara sauce, mozzarella cheese, sausage, and pepperoni, top with 1 tsp Italian seasoning, and red pepper flakes.
Loosely cover the baking dish with foil; bake on a sheet pan for dripping.
Preheat oven to 350 degrees. Bake covered, for 30 minutes. Then uncovered for 15-20 minutes or until it is well bubbly and the edges are browned. Let the lasagna rest for 10 minutes before slicing and serving.