To prepare pastry dough: Combine sifted flours, salt and oil in a mixing bowl. Pour in hot water and stir with a short wooden rolling pin to bind the dough together. Pour out dough onto a floured tabletop and mix well into a smooth dough. Leave aside to rest for a while.
Roll dough into a long strip and cut into 20 equal pieces. Roll each piece out into a circle.
Add one heaped tablespoon of cooked filling and fold into half. Pinch the edges neatly to close.
Line a steamer with banana leaf. Brush lightly with oil. Place dumplings on the banana leaf and steam for five to six minutes. Before removing the dumplings, brush with oil again. (do not over-steam or it will break.)
To prepare filling: Saute garlic and oil till fragrant. Add prawns and mushrooms, and stir-fry well. Mix in water-chestnuts and add seasoning. Stir-fry to mix then dish up to cool.
Add chopped spring onions, coriander and chillies. Mix well.