In large skillet or a wok heat the vegetable oil until hot. Add the shrimp, onion, carrot, cabbage and mushrooms and quickly stir fry while combining the ingredients. Add the pepper flakes, sesame oil, soy sauce, vinegar and sugar, adjust the seasoning and pour out onto a cookie sheet to cool. When the mixture is cool begin making the dumplings by laying out one wonton, brush the edge with water, put 1 teaspoon of the filling into the middle and fold the edge over on the diagonal to from a packet. Trim the edges to eliminate the square corners. Dust the finished dumplings with a light coating of cornstarch to prevent sticking before cooking. Refrigerate until ready to cook.
To cook the finished dumplings, bring water to a boil in a large skillet in which a steamer basket may be nestled. When the water is boiling, insert the basket with the dumplings and steam for 6 minutes or until fully cooked. The dough will change color and texture when cooked. Serve with sweet mustard dip.