Rinse and gently dry the blueberries using a paper towel.
To make the dough place the flour, salt and butter (or oil) on top of your work surface and start combining using a large knife, cutting through the mixture and gradually adding the water into the middle and gathering up the mixture with the knife to prevent the water from escaping. When all the water has been added and the dough comes together continue kneading for 6 more minutes until smooth and soft. If it sticks to your hands 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 20 minutes.
Divide the dough into 3 parts (easier to work with) and roll out one part (keeping the other 2 covered) on top of a very lightly floured surface as thinly as possible, 2-3 mm in thickness (don't worry, the dough is quite resilient and won't tear easily). Using a glass with a rim diameter of about 7 cm (or a little bigger if you prefer) make round shapes (gather up the excess dough and add to the other dough part).
Place 4-5 blueberries in the middle of each dough shape (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 removing any air bubbles - go over the edge twice for each dumpling.
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 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 (up to 15 per batch) and quickly but carefully stir with a wooden spoon. When all the dumplings have come up to the surface cook them gently for about 4 more minutes, then switch off the heat and using a slotted spoon transfer onto a large plate. Sprinkle with sugar, drizzle with a little butter and serve!
Use 4-5 blueberries in each dumpling or 1-2 more for larger dumplings. These pierogi are usually made using small blueberries but these aren't available here in the UK so I am using a large blueberry variety.
Knead the dough for about 6 minutes until very soft and smooth (this will produce softer pierogi). If the dough sticks to your hands add a bit of flour (no more than 1 teaspoon as this may make the dough a little tough).
Best served immediately. Serve with sour cream, yogurt or kefir and sugar.