Preheat the oven to 425 degrees F (220 degrees C). Grease a 9×13-inch baking dish with cooking spray. Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until tender yet firm to the bite, 8 to 10 minutes. Drain and rinse with cold water.
Mix 1/2 cup mozzarella, milk, egg, salt, and garlic salt together in a large bowl. Add drained spaghetti and mix until well combined. Spread mixture into the prepared baking dish.
Bake in the preheated oven for 15 minutes. Remove from the oven and reduce the temperature to 350 degrees F (175 degrees C). Spread spaghetti sauce over noodle mixture. Sprinkle with oregano and basil, then top with remaining 1 1/2 cups mozzarella. Arrange pepperoni over cheese.
Return to the oven and bake until cheese is bubbly and beginning to brown, about 30 minutes. Let stand for 5 minutes before cutting.
Enjoy!