To make the pastry skin: Sift the flours together and put into a mixing bowl with the salt. Stir in hot water and fat. Knead into a smooth dough. Cut into 18 to 20 even pieces. Roll out flat and wrap filling.
Place dumplings onto a piece of banana leaf and steam for 10-15 minutes.
To make filling: Shred yam beans finely. Heat oil and add dried radish, mushrooms and shredded yam beans. Stir-fry over high heat for 1-2 minutes, then add water. Season with salt, sugar and pepper to taste. Fry until vegetable turns soft and the sauce is thick. Add chopped coriander.
Dish out into a colander to drain off any excess liquid. Cool completely before use.