How to Make It
Combine watercress, chopped cilantro, grated ginger, 1 teaspoon sesame oil, chicken, and garlic, stirring well to combine.
Working with 1 wonton wrapper at a time (cover remaining wrappers to prevent drying), place wrapper on work surface, starchy side up. Moisten edges of wrapper with water. Spoon about 1 1/2 teaspoons filling in center of each wrapper, roughly shaping filling into a horizontal rectangle. Fold wrapper in half to form a rectangle, pressing well to seal edges. Moisten tips of 2 side points along folded edge, and fold over to the center of the dumpling; press well to seal. Place on a baking sheet lined with plastic wrap coated with cooking spray. Cover with a damp towel or paper towels to prevent drying. Repeat procedure with the remaining wrappers and filling.
Place stock, soy sauce, and ginger slices in a large saucepan over medium-high heat; cover and bring to a boil. Reduce heat until liquid simmers. Add dumplings; partially cover, and cook 4 minutes. Gently stir in bok choy and onions; cook 2 minutes. Remove from heat; discard ginger slices. Drizzle with remaining 1 tablespoon oil; garnish with cilantro sprigs.