Preheat oven to 375 degrees F (190 degrees C). Spray a 9x12-inch baking dish with cooking spray.
Bring a large pot of lightly salted water to a boil. Cook lasagna noodles in the boiling water, stirring occasionally, until tender yet firm to the bite, about 8 minutes. Drain.
Cook and stir sausage and garlic in a large skillet until sausage is browned, about 5 minutes. Drain excess grease. Remove from heat and stir in spaghetti sauce.
Mix ricotta cheese, egg, Parmesan cheese, parsley flakes, and oregano together in a bowl.
Spread 1/2 cup of the sauce mixture in the bottom of the prepared baking dish. Cover with 4 noodles, 1/2 of the ricotta cheese mixture, 1/3 of the mozzarella cheese, and 1/3 of the sauce mixture. Repeat layers once more. Arrange mushrooms, black olives, and pepperoni on top. Cover with remaining sauce.
Bake in the preheated oven until bubbly, about 20 minutes. Sprinkle remaining mozzarella cheese on top. Bake until mozzarella cheese is melted, about 10 minutes. Let stand for 10 minutes before serving.