In a large mixing bowl, mix the flour and salt and then add the egg, sour cream and butter. Combine the ingredients until they form a dough ball. It will be rather sticky still.
Remove the dough from the bowl and knead until the dough losses it tackiness and smooth.
Return the dough to the bowl and cover in plastic wrap and leave to stand for 30 minutes.
Whilst the dough is resting, make the filling.
Using a frying pan over a medium heat, brown the ground beef. Drain off any excess fat.
Add in onion to the frying pan and cook until translucent.
Finally, add beef stock and simmer until liquid has reduced. Season with salt and pepper.
Place to one side and allow to cool.
Now it time to make the pierogis
Place the relaxed dough on a smooth work surface. Roll the dough out to about to 1/8" thickness and using a pastry cutter, cut out 2" round circles.
Place 2 tsp of filling in the middle of each dough circle. Fold to dough in half, press out any air and pinch the edges of the pierogi together.
Make all of the pierogi before boiling.
Pour water into a large pan and bring to the boil and add salt.
Add the beef pierogi in small batches and cook until pierogi float to the top.
When the pierogi floats to the top, remove carefully.
In a large pan, add the butter and add the boiled pierogi.
Cook until lightly browned on both sides.
Melt butter in a large skillet. Serve with sour cream on warmed bowls.