In a large pan over medium high heat, add 4 tbsp of oil then add the ginger and cook for about 45 seconds. Add the chopped onions and cook until translucent (about 4 minutes)
Add the chopped shiitake mushrooms and cook for about 4 minutes or until soft. Tod the green cabbage and shredded sweet potato and stir-fry until soft (about 5 minutes) if you want to speed up the cooking process, add 2 oz water into the vegetables while you are cooking them.
Transfer the cooked vegetables into a mixing bowl and allow them to cool. Add sesame oil, soy sauce, black pepper, sherry wine, salt, sugar, and sweet potato (before adding the sweet potato you will need to smash them until smooth texture, last add the 2 oz of olive oil and mix all until well mix.
Take a wrapper and place it in the palm of your non-dominant hand. Use a teaspoon to take a small amount of filling and put it in the center of the wrapper.
Dip one finger in a bowl of water and draw a circle around the outer one-fourth” of the wrapper with your wet finger until it is wet all around.
Fold the wrapper in half over the filling.
Using your left thumb and index finger, start making a pleat about once every ¼ inch on the top part of the wrapper from the right toward the left.
Once you make each pleat, press it down with your right thumb and move toward the left
Continue all the way till there is no more top wrapper to pleat left.
Press the pleats and shape the gyoza.