Peel and boil the potatoes until fork tender. While the potatoes are boiling, make the dough.
In a food processor pulse flour and salt. With the motor running add the egg through the tube and then cold water. Let the processor do its work for a minute until the dough forms around the blade.
Transfer the dough into a bowl, cover with a tea towel and let it sit for 30 minutes.
While the dough is resting, make the filling. Frying the bacon and chop it up, then slowly cook the onion in the bacon fat over very low heat for 20 minutes until the onions are caramelised.
Mash the potatoes, add butter, bacon, onions and mix well, add salt and pepper to taste.
Divide the dough into quarters and form your quarters into balls.
Take one dough ball and roll it out on a well floured surface in a thin sheet approximately 1/16" in thickness. Keep the rest of the dough covered to avoid drying out.
Cut out circles with a 2" to 3" cookie or scone cutter. Put a teaspoon of potato filling into each dough circle, slightly off-centre, fold the dough over to form a half-moon shape and pinch the edges shut with your fingertips. If you want a more attractive look go over the edge one more time and this time pinch the edges together using your two fingers and a thumb and twist them to form a ruffled edge.
Repeat with the remaining dough circles until you run out of both dough and the filling. You should have about 65 vareniki.
Set aside a needed amount of pelmeni for dinner and freeze the rest in a well floured and air-tight container to prevent sticking.
Bring a large pot of water to a boil, add salt drop vareniki into rapidly boiling water and stir to prevent them sticking to the bottom, once they float to the top, drain and pan fry in butter over medium heat until golden on both sides.
Serve with sour cream, dill and crispy fried onions or shallots.
*Cook the remaining vareniki from frozen.