"I love my Philly cheesesteak sandwiches with marinara, so I thought I'd try making lasagna! It turned out fantastic! Note that you do not have to boil your noodles for lasagna. Just let them soak in hot tap water for at least 20 minutes."
Combine flank steak, onions, green bell peppers, and mushrooms in a large resealable plastic bag. Add Worcestershire sauce, olive oil, salt, and pepper. Seal and let marinate in the refrigerator, about 30 minutes.
Preheat oven to 350 degrees F (175 degrees C).
Place lasagna noodles in a 9x13-inch baking dish. Cover with hot water and let soak until tender, about 20 minutes. Drain in a colander. Grease baking dish with cooking spray.
Transfer steak to a large baking sheet. Arrange onions, green bell peppers, and mushrooms around steak. Discard marinade.
Bake steak in the preheated oven until starting to brown, 15 to 20 minutes. Turn on broiler; broil until steak darkens, about 5 minutes.
Combine spaghetti sauce and garlic powder in a large skillet over medium-low heat. Slice steak into thin pieces. Stir into the spaghetti sauce. Add onions, green bell peppers, and mushrooms. Remove from heat.
Mix ricotta cheese, half of the mozzarella cheese, Parmesan cheese, egg, and parsley together in a bowl.
Spread 2 cups of the sauce mixture in the bottom of the baking dish. Top with 4 lasagna noodles. Cover noodles with 1/3 of the ricotta mixture, 2 cups of the sauce mixture, and 1/3 of the provolone cheese. Repeat layers twice. Sprinkle remaining mozzarella cheese on top. Cover baking dish with greased aluminum foil.
Bake in the preheated oven until bubbling, about 45 minutes. Uncover and continue baking until mozzarella cheese is browned, about 15 minutes. Let stand for 15 minutes before cutting and serving.