To prepare sauce, combine first 4 ingredients; set aside.

To prepare filling, heat a large non-stick skillet coated with cooking spray over medium-high heat. Add mushrooms; cook 3 minutes or until moisture evaporates, stirring frequently. Add cabbage; cook 3 minutes or until softened, stirring frequently. Stir in 2 tablespoons green onions and next 6 ingredients (2 tablespoons green onions through garlic); simmer 2 minutes. Remove from heat; set aside.

Working with 1 wrapper at a time, spoon 2 teaspoons mushroom mixture into center of wrapper. Fold in half. Fold top edge of wrapper at 1/2-inch intervals to form pleats, pressing against bottom edge to seal. Place dumplings, pleated sides down, on a large baking sheet dusted with flour; cover loosely with towel to prevent drying.

Line each tier of a 2-tiered (10-inch) bamboo steamer with 3 lettuce leaves. Arrange 6 dumplings, 1-inch apart, in each steamer basket. Stack tiers; cover with steamer lid. Add water to a large skillet to a depth of 1 inch; bring to a boil. Place steamer in pan, and steam dumplings 5 minutes. Remove dumplings from steamer. Repeat procedure with remaining lettuce and dumplings.

Heat 1 1/2 teaspoons oil in a large non-stick skillet over medium-high heat. Add 12 dumplings; cook 1 1/2 minutes on each side or until browned. Repeat procedure 3 times with remaining oil and dumplings. Serve with sauce.