Place mushrooms in a heatproof bowl. Cover with boiling water. Set aside for 30 minutes or until soft. Drain. Remove and discard stalks. Finely chop.
Place spinach in a microwave-safe dish. Cover. Microwave on HIGH (100%) for 2 to 3 minutes or until wilted. Place in a sieve. Using the back of a large spoon, press spinach to drain excess liquid. Transfer to a chopping board. Roughly chop.
Combine mushroom, spinach, water chestnuts, oyster sauce, sesame oil and garlic in a bowl. Place 1 wrapper on flat surface. Spoon 2 teaspoons mushroom mixture on 1 half of wrapper. Brush edges with cold water. Fold over to enclose filling. Press edges together and pleat to seal. Place on a tray lined with baking paper. Repeat with remaining wrappers and mushroom mixture.
Place a large bamboo steamer lined with baking paper over a large saucepan or wok of simmering water (make sure it doesn’t touch water). Using a metal skewer, pierce holes in baking paper. Steam dumplings, in batches, for 10 to 12 minutes or until cooked through. Serve with soy sauce.