Serve immediately warm or hot, with sour cream and, maybe, a little ground black pepper on top. We also add a drop of vinegar and a little salt, that's how we used to serve them at home.
By the way, if you want to reheat the cold dumplings, you can fry them. Place them in a pan and cook, carefully mixing until they brown nicely. My husband loves them even more on the second day, cooked like this.