For the dough: In a large bowl, combine the flour and salt. Make a well at the centre and pour in the sour cream and egg. Mix with a fork, gradually adding the hot water, until the dough starts to come together.
Place the dough on a lightly floured work surface and knead until smooth. Shape into a disc with your hands. Cover in plastic wrap and let rest for 30 minutes at room temperature.
For filling: In a bowl, combine the mashed sweet potatoes with the cottage cheese. Season with salt and pepper. Set aside.
Assembly: On a floured work surface, roll the dough out until about 1/16 inch (2 mm) thick. Using a round 3-inch (7.5 cm) cookie cutter, cut out 24 circles of dough. Place about 2 tsp (10 ml) of the filling at the centre of each circle. Brush the edges of the dough with water and fold over into halfmoons, pressing the edges together to seal. Set aside on a lightly floured baking sheet.
In a non-stick skillet over medium-high heat, cook the onions and bacon in a small amount of the butter, as needed, until golden. Season with salt and pepper. Set aside and keep warm.
In the same skillet, heat 1 tbsp (15 ml) of the butter. Place half of the pierogi in the skillet, without letting them touch. Pour in enough water to come 1/2 inch (1 cm) up the side of the skillet. Cover. Bring to a boil and cook for 5 minutes. Remove the lid and cook for another 2 minutes or until the water has completely evaporated and the pierogi are starting to brown. Flip the pierogi over. Add 1 tbsp (15 ml) of the butter and cook just until the pierogi start to brown on the second side. Keep warm on a large plate. Continue cooking the remaining pierogi in the remaining butter.
Top the pierogi with the cooked onion mixture. Serve with sour cream or cottage cheese. Sprinkle with chives. Delicious served with sauerkraut.