01 - In a large mixing bowl, combine ground beef, chopped shiitake mushrooms, spring onions, garlic, ginger, soy sauce, oyster sauce, sesame oil, sugar, white pepper, and salt. Mix thoroughly to evenly blend the ingredients.
02 - Place one dumpling wrapper flat in your palm. Add one heaping teaspoon of filling in the center, moisten the edge with water, fold over, and pleat to seal, ensuring an airtight closure. Repeat until all filling is used.
03 - Arrange dumplings in a single layer inside a parchment-lined steamer basket, leaving small gaps between each. Steam over gently simmering water for 8 to 10 minutes, until the beef is fully cooked.
04 - Heat 2 tablespoons vegetable oil in a large nonstick skillet over medium-high. Place dumplings flat side down and sear until golden brown on the bottom, about 2–3 minutes. Add 1/4 cup water, cover, and steam for 5–6 minutes or until water has evaporated. Remove lid and continue cooking for 1 more minute for extra crispiness.
05 - In a small bowl, blend soy sauce, rice vinegar, sesame oil, and chili flakes if desired. Stir well to combine.
06 - Serve dumplings hot, accompanied by the prepared dipping sauce.