You can get gyoza wrappers at an Asian supermarket. They are usually sold frozen. Ideally you should thaw them overnight in the fridge, which prevents them from getting soggy. If you're in a rush, thaw them on the kitchen counter - this will take about 1-2 hours. Once you open the wrapper package, keep it covered by a moist towel at all times as they tend to dry out very quickly (which makes them much harder to handle and less tasty once cooked).

Blanch the cabbage leaves in salt water for a minute, then transfer to a bowl of ice water for a few seconds. Dab dry with a paper towel. Cut off the core (the hard stem in the middle) of the cabbage leaves. Chop the leaves into very small pieces.

Finely chop the green onions and shiitake mushrooms. Combine the meat, cabbage, green onions and shiitake mushrooms in a large bowl. Add the minced garlic and grated ginger. Add the seasonings: sesame oil, soy sauce, sake, salt and pepper. Mix well and knead the mixture by hand until it gets paler and sticky.

Take a wrapper and place it in the palm of one hand. Add a small amount of filling into the center of the wrapper using a teaspoon. Dip one finger in a glass of water and draw a circle along the outer edge of the wrapper so that it's wet all around (this will act as 'glue').

Take a wrapper and place it in the palm of one hand. Add a small amount of filling into the center of the wrapper using a teaspoon. Dip one finger in a glass of water and draw a circle along the outer edge of the wrapper so that it's wet all around (this will act as 'glue')

Fold the wrapper in half and pinch it just in the center with your fingers (without sealing the dumpling completely!). Using your thumb and index finger, start folding pleats, starting from the middle (see picture). Work yourself to one edge, then start in the middle again and work yourself to the other edge. Press each pleat to the back side of the wrapper with your fingers to seal the dumpling. Folded gyoza should be cooked or frozen right away as they can get soggy otherwise.

Heat oil in a non-stick frying pan over medium heat. Place the gyoza, flat side down, with a little bit of space in between each piece, into the pan. (In rows or in a circular shape). Cook until the bottom of the gyoza is golden brown (ca. 3 min). Add 1/4 Cup of water to the pan and immediately cover with a lid. Let the gyoza steam for 3 min or until most of the water has evaporated. Remove the lid and drizzle 1 tsp sesame oil along the pan. Cook uncovered until gyoza are crisp on the bottom.

To freeze the gyoza, transfer them on a tray lined with a baking sheet immediately after folding them. Leave space in between so they don't stick. Put the tray in the freezer. Once the gyoza are frozen, transfer to a freezer bag or Tupperware. When using them later on, DO NOT defrost: cook while frozen as described above and steam for additional 1-2 min.