Combine first 10 ingredients in a food processor, and process until finely chopped.
Heat a large nonstick skillet over medium heat. Coat pan with cooking spray. Add mushroom mixture and spinach to pan; cook 10 minutes or until liquid evaporates, stirring frequently. Cool slightly.
Moisten edges of gyoza skin with water, working with 1 gyoza skin at a time (cover remaining skins to prevent drying). Spoon about 1 tablespoon spinach mixture into center of circle. Fold in half, pinching edges together to seal. Place dumpling, seam side up, on a baking sheet (cover loosely with a towel to prevent drying). Repeat procedure with remaining wrappers and filling.
Heat 1 teaspoon oil in a large nonstick skillet over medium-high heat. Add half of dumplings to pan; cook 2 minutes on each side or until lightly browned. Remove from pan; keep warm. Repeat procedure with remaining 1 teaspoon oil and remaining dumplings. Add 1/4 cup broth, 1/3 cup juice, 1 tablespoon soy sauce, and vinegar to pan; bring to a boil. Place dipping sauce in a small bowl.
Add remaining 1/4 cup broth to pan; bring to a boil. Add dumplings; cover and cook 2 minutes or until tender. Remove from pan. Serve with dipping sauce; top with chives, if desired.