In the bowl of an electric mixer, combine all of the ingredients for the dough. Wrap in plastic wrap and let rest in the refrigerator. For easier handling, as it is a sticky dough, you could also place in the freezer for 10 minutes prior to using.
Roast the sweet potato whole until fork tender along with a small onion, sliced and tossed with about 1 tsp of oil. This will be added to your sweet potato. You could also peel, dice and boil them. You'll just have to saute the onion separately and caramelize in 1 tsp of oil. Allow to cool.
Meanwhile, in a large saute pan with the oil hot, add the onions and cabbage using the water after they start to soften to allow the mixture to steam. Saute until somewhat tender. Add the rest of the cabbage mixture ingredients & continue to saute until tender. Set aside.
In the bowl of a food processor, add the cooled sweet potato and onion. Add the remaining ingredients for the sweet potato mixture and process until almost smooth. Taste for seasoning, add more salt and pepper if necessary.
Using a well-floured surface and rolling pin, roll dough to about 1/4 inch thick. Allow to rest for a few minutes. Using a large round cookie cutter, cut circles and place about 1 Tbl of filling in the center of each. Fold over and press to seal. Using a fork, crimp edges and use the cookie cutter if you need to trim up any excess. The most important thing is to be sure they are sealed or they will seep out during the boiling process. Place on a lined sheet pan as you make them and complete all before proceeding.
Heat a pot of salted water to boiling. Gently place a few pierogi at a time in the water. They will float to the top when they are ready. Remove to a towel-lined cookie sheet to drain.
Heat a skillet on high heat and add 1 tsp of oil. Add the drained pierogi and quickly brown on both sides.
Serve with the cabbage mixture and a side of sour cream.
Garnish with parsley and a few more caraway seeds.