1. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add mushrooms to pan; sauté 8 minutes or until liquid evaporates. Spoon mushrooms into a food processor. Add onions and next 5 ingredients (through 4 ounces shrimp); process 10 seconds or until finely chopped.
2. Working with 1 gyoza skin at a time (cover remaining skins to prevent drying), spoon about 1 tablespoon shrimp mixture into center of each skin. Moisten edges of skin with water. Gather up and crimp edges of skin around filling; lightly squeeze skin to adhere to filling, leaving top of dumpling open. Place 1 shrimp piece on top of filling, pressing gently into filling. Place dumpling on a baking sheet; cover loosely with a damp towel to prevent drying. Repeat procedure with remaining skins and filling.
3. Line each tier of a 2-tiered bamboo steamer with 2 cabbage leaves. Arrange 12 dumplings, 1 inch apart, over cabbage in each steamer basket. Stack tiers, and cover with steamer lid.
4. Add water to skillet to a depth of 1 inch, and bring to a boil. Place steamer in pan, and steam dumplings for 15 minutes or until done. Remove dumplings from steamer, and spoon 1/8 teaspoon sambal oelek onto each dumpling. Discard cabbage.