Add flour and salt to a bowl. Boil water with a water kettle and slowly pour into the flour whilst constantly whisking with a kitchen machine using a bread hook. Mix or whisk for about 5-10 minutes or until the dough sticks to the bread hook and has an elastic texture.
Wrap the dough in a damp kitchen towel and put it into th fridge for 30 minutes.
In the meantime you can prepare the filling. Rinse off shiitake and cut them into small pieces.
Thinly slice leek, rinse it off and cut in smaller pieces if necessary.
Heat a pan and roast chili flakes in the pan until they start to smell. Put aside for later.
Heat sesame oil in a pan, add leek and shiitake and sauté for about 3-4 minutes. Deglaze by adding soy sauce and minrin.
Add garlic ginger paste and mix it with the filling. Cook for another 2 minutes, then remove from the heat.
Add roasted chili flakes and season with salt. Mix everyything together and put aside.
Dust a flat surface with starch. Thinly roll out the dough. Make sure it doesn`t stick to the surface by turning and moving it from time to time.
Use a round cookie cutter or a glass to cut out dough circles. Please note if you`re cutting out the whole dough at once, don`t place the dough circles on top of eachother, as they will stick together. It`s better to cut out a few, fill and fold them and then do another round.
Place 1 tsp of filling in the middle of a dough circle, moist the edges by diping your finger into water and running it across the outer line of the circle. Fold into a halfmoon sticking together the middle, then form fan-like folds from the middle to the right edge and same to the left edge. Repeat this step for every dumpling and line them on a baking sheet covered with parchment paper.
Heat a pan that you can cover, add oil and place dumplings with the bottom into the pan. Pan sear for about 5 minutes or until they start to brown on their bottom.
Add about 150ml of water to the pan, cover with the lid and let them steam for about 5 to 7 minutes.
Remove lid and go on cooking the gyoza until all the water is evaporated.
Dip the fried bottom of your gyoza into sesame seeds and serve the potstickers with soy sauce for dipping.