Boil the potatoes in salted water, drain well and mash until smooth. Set aside to cool completely. You can do this step in advance. Fill a large pot with water, add salt, cover and bring to boil. When the water starts boiling your kopytka will be ready to go in the pot. Combine the mashed potatoes with the egg, season and start adding the flour, mixing in with the back of a spoon.
When you've added most of the flour scatter the rest over your work surface and place the dough mixture on top. Knead for a few more seconds to incorporate all the flour. Divide the dough into 3-4 parts (easier to work with). The dough can be a little sticky. Roll each part into a long sausage shape. Cut into 2 cm thick pieces (straight or at an angle). If the dough sticks to the surface dust with a little flour (5 g should be enough).
When the water starts to boil put the dumplings into the pot, one by one, and gently stir to prevent them sticking to the pot and each other. Once they all come up to the top boil them gently for 2 more minutes. Turn off the heat, lift the dumplings out using a slotted spoon and place on a large plate. Add a knob of butter or drizzle of oil to prevent them sticking to one another. Serve immediately. See also Serving Suggestions in post.
To make the quick mushroom sauce fry the mushrooms over a fairly high heat in 2 tablespoons of oil for 5 minutes, stirring occasionally. Dust with the flour and stir in to combine. Add the water, stir and cook until the sauce thickens (add a little more water if you prefer). Remove from the heat, stir in the fromage frais and season to taste. Serve with the dumplings.
Potatoes: Use Maris Piper, Yukon Gold or another all-rounders variety. Drain the cooked potatoes well and mash until smooth. Cool completely before making the recipe. You can boil and mash the potatoes in advance and once cooled refrigerate overnight.
Dough: Try not to knead the dough more than is necessary. Stop kneading once all the flour has been mixed in.
Dumpling shapes: You can cut the sausage shaped dough at an angle or straight (the photos show both methods).
Make it vegan: Use 2 teaspoons of ground flaxseed instead of the egg. You could also omit the fromage frais in the mushroom sauce and just use a bit more water.
Boiling the dumplings: Do not overcrowd the pot. Make the dumplings in batches if you have to (in the same water).
Best served immediately - see post (above) for alternative serving suggestions.
Freezing: To freeze cooked dumplings thoroughly coat in oil and freeze individual portions. To freeze raw dumplings place on a flour dusted board and freeze. Then store in a freezer bag and place back in the freezer. Freeze for up to 3 months.
Preparation time includes the time needed to boil and cool the potatoes.