Bring a large pot of lightly salted water to a boil. Add spaghetti and cook for 8 to 10 minutes or until al dente; drain and cool.
Beat egg in large bowl, add milk, 1/2 cup of the mozzarella cheese, salt and garlic salt. Stir in cooked spaghetti, mix well.
Spread mixture evenly in greased 20x30cm baking dish. Bake for 15 minutes. Remove from oven and reduce temperature to 180 degrees. Spread sauce over spaghetti. Sprinkle with oregano, basil and the remaining 1 1/2 cups mozzarella cheese. Top with pepperoni, return to oven and bake for 30 minutes. Let stand 5 minutes before cutting.