My family makes pierogi every Christmas as one of the traditional courses of the Wigilia dinner on Christmas Eve. The filling that my aunt always uses is sauerkraut and mushroom (my favorite), so I’ll add it here as an additional filling option for the recipes above.
1 and a half lbs. sauerkraut
2 large onions
2 lbs. mushrooms
Boil the sauerkraut for 30 minutes in a little water, letting the water evaporate. Mince the sauerkraut and sauté one onion until it is golden brown, then mix the kraut with the onion. Slice the mushrooms and cook them with 2-3 spoons of water, until the water evaporates. Chop the second onion and sauté like the first, add the mushrooms to this and season the mixture with salt and pepper before mincing and then mixing with the sauerkraut.
Boiling the sauerkraut takes away some of it's sourness, resulting in a nice mild flavor, made rich and earthy by the addition of the onion and mushrooms. The butter that the dumplings are fried in right before serving is also very important.