How to Make It
Remove stems from mushrooms; discard or reserve for making stock. Finely chop mushroom caps.
Heat a large skillet over medium-high. Add oil; swirl to coat. Add mushrooms, leek, ginger, and garlic; cook until leek is tender and mushroom liquid has evaporated, about 7 minutes. Cool 10 minutes.
Line a baking sheet with plastic wrap, and coat with cooking spray. Combine chicken, mushroom mixture, and 1/2 teaspoon soy sauce. Working with 1 wonton wrapper at a time (cover remaining wrappers with a damp towel to prevent drying), place wrapper on a work surface, starchy side up. Moisten edges of wrapper with water. Spoon about 2 teaspoons filling into center of each wrapper. Bring 2 opposite corners together; pinch points together to seal. Bring remaining 2 corners to center; pinch to seal. Pinch 4 edges together to seal. Place on prepared baking sheet; cover with a damp towel or paper towels to prevent drying. Repeat procedure with remaining wrappers and filling.
Line both tiers of a 2-tiered bamboo steamer with parchment paper. Divide dumplings and bok choy between baskets. Stack baskets; cover with steamer lid. Add water to a skillet to a depth of 1 inch; bring to a boil over high. Place steamer in skillet; steam dumplings 8 minutes or until done.
Meanwhile, combine lemon juice, mirin, sugar, and remaining 2 tablespoons soy sauce in a small bowl, stirring until sugar dissolves. Serve dumplings and bok choy with sauce.