Mix the prawn meat, sprig onions, ginger and water chestnuts in a bowl. Put the corn-flour, sesame oil, soy sauce, sugar, salt and pepper in another bowl; blend to a smooth paste, then stir into the prawn mixture.
Work with one dough round at a time and keep the rest covered. Hold a dough circle in the palm of your hand (this will keep the pastry warm) and put a rounded teaspoon of filling in the centre. Bring up two sides to meet and press the edges to seal. Press the gow gee firmly down on the bench to make a flat base and twist both sides down to form a crescent. Brush very lightly with sesame oil. Place on an oiled baking tray.
Line the base of a bamboo or metal steamer with a circle of baking paper. Arrange the gow gees on the paper, spacing them well apart. (They may need to be cooked in batches.) Cover the steamer and cook over a pan of simmering water for about 8 minutes or until the dough is slightly puffy and translucent. Serve immediately with sesame oil mixed with soy sauce, if desired.