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.
We couldn't find gyoza skins, so we went with wonton, and ended up with 35 dumplings instead of 28. And they were DELICIOUS! Mmm, would make again. Just be aware--there's a fair amount of "hands on" time for this recipe, so save it for a day where you have more than 30 minutes to get dinner ready.
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