Combine flank steak, onions, 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 for about 30 minutes.
Meanwhile, place lasagna noodles into a 9x13-inch baking dish. Cover with hot water and let soak until tender, about 20 minutes. Drain noodles in a colander, then dry the baking dish and grease with cooking spray.
Preheat the oven to 350 degrees F (175 degrees C).
Transfer marinated steak to a large baking sheet and discard marinade. Arrange onions, bell peppers, and mushrooms around steak.
Bake in the preheated oven until steak starts to brown, 15 to 20 minutes. Turn on the broiler and broil until steak darkens, about 5 minutes. Remove from the oven and return the heat to 350 degrees F (175 degrees C). Slice steak into thin pieces when cool enough to handle.
Combine spaghetti sauce and garlic powder in a large skillet over medium-low heat. Stir in sliced flank steak. Add onions, bell peppers, and mushrooms. Remove from the heat.
Mix ricotta, 1/2 of the mozzarella, Parmesan, egg, and parsley together in a bowl.
Spread 2 cups sauce mixture in the bottom of the prepared baking dish. Top with four lasagna noodles. Cover noodles with 1/3 of the ricotta mixture, 2 cups sauce mixture, and 1/3 of the provolone. Repeat layers twice. Sprinkle remaining mozzarella on top. Cover the 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.