In a small bowl, mix the kefir with the water.
In a large bowl, whisk the egg with a fork, then stir in the kefir-water mixture.
Sift the flour together with a pinch of salt. Gradually add the flour to the wet mixture, first mixing with a fork, then kneading by hand.
Note: The dough will feel stiff—don’t worry! Knead it for 5–10 minutes until smooth. Shape it into a ball, wrap tightly in plastic wrap, and let it rest at room temperature for 40 minutes. This step is essential, as the dough will become softer and easier to work with.
After resting, cut off a small portion of dough and keep the rest wrapped. Lightly flour your work surface and roll out the dough thinly. Use a glass to cut out circles, and re-wrap the scraps between batches to prevent drying out. (Tip: Use as little flour as possible when rolling—just enough to prevent sticking.)
To assemble: Place a few blueberries in the center of each dough circle, sprinkle with sugar, and fold the dough over. Pinch the edges tightly to seal and form dumplings. Prepare all the dumplings this way.
Arrange the dumplings on a floured board, making sure they don’t touch. If you want to enjoy the dumplings later you can freeze them at this point: place the board in the freezer for 2–3 hours, then transfer the frozen dumplings to a sealed bag for storage. To cook: Bring a large pot of salted water to a boil. Reduce to a simmer and add the dumplings. Once they float to the surface, cook for another 2–3 minutes. Remove with a slotted spoon.
Serve warm, sprinkled with sugar and a generous dollop of fresh sour cream.
Enjoy!