Peel and boil the potatoes, covered, in salted water. Drain thoroughly, mash and leave to cool.
Heat 1 tablespoon of oil and fry the onions slowly until softened for 5 minutes stirring often (don't brown onions). Combine with the potatoes. Once cooled add cottage cheese, season well and mash using a potato masher for a smooth finish.
To make the dough place flour, salt and butter combining using a large knife, cutting through the mixture and gradually adding water into the middle and gathering up the mixture with the knife. When all water has been added and dough comes together continue kneading for 6 more minutes until smooth and soft. If it sticks to your hands add a bit of flour (no more than 1 tsp) and knead in. Cover with a bowl and set aside for 20 minutes.
Divide 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. Using the rim of a glass with a diameter of about 7 cm cut out round shapes (gather up the excess dough and add to the other dough part).
Place a small amount of the filling (about 1 teaspoon) in the middle of the round 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, push the filling away from the edges and remove any air bubbles - go over the edge twice for each dumpling (your fingertips should be dry to avoid sticking)
Place the dumplings on a lightly floured surface and cover with a 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 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. Drizzle with a little butter and serve with sour cream.