01 - In a large mixing bowl, combine the ground beef, finely chopped napa cabbage, onion, green onions, minced garlic, grated ginger, soy sauce, toasted sesame oil, mirin, sugar, black pepper, and egg. Mix thoroughly until all ingredients are evenly incorporated and the mixture holds together.
02 - Place a dumpling wrapper in the palm of your hand and spoon approximately 1 rounded teaspoon of filling into the center. Dip your finger in water and moisten the entire edge of the wrapper. Fold the wrapper over into a half-moon shape and pinch the edges firmly to seal, creating pleats along the edge if desired. Repeat with the remaining wrappers and filling.
03 - In a small saucepan, combine the gochujang, honey, soy sauce, rice vinegar, sesame oil, and minced garlic. Place over low heat and simmer for 2 to 3 minutes, stirring occasionally, until the glaze thickens slightly and becomes glossy. Remove from heat and set aside.
04 - Heat 1 tablespoon of vegetable oil in a large nonstick skillet over medium-high heat. Arrange the filled mandu in a single layer without overcrowding. Cook for 2 to 3 minutes until the bottoms develop a deep golden crust.
05 - Carefully pour ¼ cup of water into the skillet and immediately cover with a lid. Steam for 3 to 4 minutes until the wrappers are translucent and the filling is cooked through. Remove the lid and continue cooking for 1 to 2 additional minutes until all the water evaporates and the bottoms regain their crispness.
06 - Transfer the cooked mandu to a serving plate. Drizzle or brush generously with the warm gochujang glaze. Sprinkle with toasted sesame seeds and serve immediately while crisp.