Preheat oven to 450 degrees F (230 degrees C). Line a baking sheet with foil and spray with cooking spray.
Grate cauliflower in a food processor.
Put a steamer in a saucepan, add water to reach just below bottom, and bring to a boil. Add cauliflower and steam, covered, until tender, 8 to 10 minutes. Transfer to a large plate, spreading it out. Refrigerate, stirring occasionally, until cooled, about 15 minutes. Transfer to a smooth kitchen towel (not terry cloth), gather up towel ends, and twist, firmly squeezing out as much liquid as possible.
Mix together egg, cheese, parsley, garlic, salt, and pepper in a bowl. Stir in cauliflower with a fork until evenly incorporated. Mound mixture in center of baking sheet; press and shape into a 12-inch round (about 1/4 inch thick).
Bake until lightly browned, 15 to 18 minutes.
Spread tomato sauce on crust and add pepperoni (if using), then sprinkle with Italian and parmesan cheeses. Bake until cheese is melted and bubbling, about 5 minutes. Scatter with basil.