Mandu (Korean dumplings)

Name: Mandu (Korean dumplings)
Cuisine: Korean
Ingredients: 1 package dumpling skins/wrappers (about 40 pieces), 만두피 (mandu pi); 8 ounces zucchini (finely chopped); 10 ounces green cabbage (finely chopped); 4 ounces fresh mushrooms (finely chopped (shiitaki preferably)); 1/2 medium onion (finely chopped); 2 scallions (finely chopped); 1/2 pound ground pork (or other meat if preferred); 1/4 pound ground beef; 1 tablespoon minced garlic; 1 to 2 teaspoons finely minced ginger (or juiced); 1 tablespoon soy sauce; 1 tablespoon sesame oil; 1 egg; 1/4 teaspoon salt to season the filling and more for salting vegetables; 1/8 teaspoon pepper; 1 tablespoon soy sauce; 1 teaspoon vinegar; 1 tablespoon water; 1/2 teaspoon sugar; pinch of black pepper; pinch of red pepper flakes (gochugaru)