For the crust, put flour and salt into a mixing bowl and rub in butter with a pastry cutter until mixture resembles fine breadcrumbs.
Add cold water and mix to form a firm dough. Place dough in a plastic bag and chill for 20 minutes.
Roll out the pastry to line a 20cm nonstick spring-form pie pan. Trim off the excess and prick the base with a fork. Line with greaseproof paper. Fill with red or green beans. Bake in a preheated oven at 190°C for 15 minutes then remove paper and beans.
Meanwhile, heat a frying pan and melt butter. Add onion and leek; sauté until fragrant. Add chopped spring onion and fry for 5-10 minutes or until softened. Set aside. Transfer filling to the crust.
Combine beaten eggs, whipping cream, fresh milk, salt and pepper. Mix until well blended. Pour into the crust and sprinkle with grated mozzarella cheese.
Bake in preheated oven at 180°C for 30-35 minutes or until set. Cool for 5-10 minutes.