Finely chop the cabbage, carrots, celery, and green onions. Grate the ginger and mince the garlic and chili pepper. In a large mixing bowl, combine the ground chicken, chopped vegetables, soy sauce, grated ginger, minced garlic, and chili pepper. Mix until well combined.
In another mixing bowl, combine the flour and salt. Gradually add hot water, mixing until the dough comes together. Knead on a floured surface until smooth and elastic, about 5-7 minutes. Cover and let rest for about 30 minutes.
Divide the dough into small pieces, roll each into a thin circle. Place a spoonful of filling in the center, fold and pinch the edges to seal.
Prepare your steamer, bring water to a boil. Place dumplings in the steamer, cover, and steam for about 15-20 minutes, until cooked through.
In a small saucepan, combine soy sauce, lemon juice, honey, rice vinegar, garlic, ginger, chopped green onion, and optional chili pepper. Heat until warm, then stir in sesame oil.
Transfer cooked dumplings to a serving platter. Serve warm with the soy dipping sauce on the side.