Pour enough hot water to cover bean threads and dried mushroom. Let stand 30 minutes to soften. Stir fry chicken in oil 2-3 minutes until no longer pink. Push to side of wok. Add onion and garlic; stir fry 1 minute. Remove from heat. Stir crabmeat into chicken mixture with fish sauce. Drain bean threads and mushrooms; finely chop and add to chicken mixture. Makes enough filling for 9 spring rolls (18 halves).