1. Heat 1 tablespoon vegetable oil in a large saute pan or wok over high heat. Saute cabbage, stirring occasionally, until translucent, 2 to 3 minutes. Season with 1/2 teaspoon salt, and transfer to a large bowl. Heat another tablespoon vegetable oil, and saute onion, stirring occasionally, until just softened, 2 to 3 minutes. Season with 1/2 teaspoon salt, add to bowl with cabbage, and let cool 5 minutes. Process in a food processor until just coarsely chopped, and return to bowl.

2. Heat 2 teaspoons vegetable oil in pan over medium heat, and saute chives until just tender, about 1 minute. Transfer to chopped cabbage mixture, and let cool. Stir in scallions, garlic, pork, sesame seeds, sesame oil, 3/4 teaspoon salt, and 1/4 teaspoon pepper.

3. Stir together soy sauce, vinegar, and remaining 1/4 teaspoon pepper in a small bowl; set dipping sauce aside.

4. Working with 1 wrapper at a time, moisten edge with egg whites, then place 2 teaspoons filling in center. Fold in half to form a half-moon, press edge to seal tightly, and place on a parchment-lined rimmed baking sheet.

5. Wipe out pan. Working in batches and wiping out pan between each, heat 1 tablespoon vegetable oil until shimmering, and sear dumplings until browned, about 1 minute per side. (If preparing in advance, stop here and steam before serving.) Add 1/4 cup water, and cover with a tight-fitting lid. Steam dumplings 2 minutes. Transfer to a platter, and loosely tent with foil to keep warm. Repeat with rest of dumplings. Garnish with scallions and sesame seeds. Serve dumplings with dipping sauce.