Shell, devein, wash, and drain prawns. Dice into bits.
Boil mushrooms in water for 10 minutes, rinse, squeeze dry, cut off and discard stems; then chop into very small pieces.
Combine the pork, mushrooms, prawns, and onion. Put mixture on chopping board and chop 10 to 15 strokes with cleaver. (Use a sharp knife if you don't have a cleaver.) Texture, when you're finished, should be slightly finer than hamburger.
Add "seasoning" and the egg to the pork mixture. Mix well.
To make dumpling, place 1 Tb filling in the center of a dumpling skin. Then bring all sides of the skin up to cover the meat as much as possible, without closing. The top of the dumpling is left open.
Cook dumplings by steaming for 30 minutes. Use as many as you need, with the rest, cool, wrap, freeze. Reheat after thawing by steaming 10 minutes.
Serve with soy sauce, hot sauce, or mustard.
* Dumpling skins are similar to won ton skins, except that they are round and slightly thinner. You may substitute won ton skins by merely cutting off the corners to round off the skin.