To make the filling place the mushrooms in a bowl, add a little water and rub using your fingertips to remove any grit. Rinse well, place in a pot, add fresh water (enough to cover the mushrooms thoroughly) and soak for 10 minutes. Cover, bring to the boil, then simmer for 30 minutes.
To make the dough place the flour, salt and butter on top of your work surface and start combining using a large knife, cutting into the mixture and gradually adding the water into the middle, gathering up the mixture with the knife to prevent the water from escaping. When the dough starts to come together and all the water has been added knead the dough for about 6 minutes until it becomes smooth and soft. If it sticks to your hands too much add a bit of flour (not more than 1 tsp) and knead in. Cover with a bowl or wrap in cling film and set aside for 15 minutes.
Heat up the oil and fry the onion over a low heat for 2-3 minutes until it's softened, stirring occasionally (do not brown it as it may taste bitter). Set aside.
Once the mushrooms have cooked strain well reserving the liquid (which you can add to Polish borscht or krupnik soup). Chop the mushrooms roughly, combine with the onion, soy sauce and seasoning and puree until the mixture becomes sticky (it can still have bigger bits of mushrooms or onion in it). Add the breadcrumbs and stir thoroughly. Taste the mixture and adjust the seasoning if needed.
Divide the dough in 2 or 3 parts (easier to work with) and roll out one part (keeping the other 2 covered) on top of a lightly floured surface as thinly as possible (don't worry, the dough is stretchy and quite resilient and won't tear easily). Using the rim of a glass with a diameter of approximately 6 cm make round shapes (gather up excess dough and combine with the other dough part).
Place a small amount of the filling in the middle of each dough circle (if the dough is sticking to the work surface use a knife to help it come off but take care not to tear it). With tips of your fingers stick the edges of the dumpling together and bring the ends towards each other and stick together too (your fingertips should be dry so it's a good idea to keep a sheet of paper towel near you to wipe them if they get sticky).
Place the dumplings on a lightly floured surface and cover with a tea towel.
Fill a large pot with salted water, add 1 tsp of oil, cover and bring to the boil. Continue making the dumplings as you are waiting for the water to boil. When the water starts boiling carefully place the dumplings in the pot one by one (don't overcrowd the pot, do this in batches if you have to) and quickly but carefully stir with a wooden spoon. When the dumplings come up to the surface cook them gently for about 4 minutes, switch off the heat and using a slotted spoon transfer them onto a lightly greased large plate.
Serve hot along with Christmas borscht, 4-5 per one portion of the borscht.
The most affordable porcini mushrooms I've ever come across are sold in Polish delicatessens. Avoid big supermarkets.
You can make the filling ahead and once cooled refrigerate for up to 2 days.
Make sure you stick the edges of each dumpling together really well so they don't open up in the pot.
Make ahead: If you don't want to serve the dumplings straight away place them in an oven proof dish (the oil will prevent them from sticking to one another) and once cooled refrigerate (for up to 3 days) until they are ready to be reheated (in the same dish). Cover with tin foil and reheat in the oven. Alternatively reheat individual portions in the microwave.