To make potato filling: Place potatoes in a saucepan large enough to cover with water and bring to boil. Boil until potatoes are fork tender. Rinse under cold water and peel skins off at the same time. Place potatoes back into drained saucepan and mash with masher. Add butter, cheese and salt and pepper. Whip until smooth but not soft. Let cool before using in pierogis.
To make cabbage filling: Place ingredients into a large skillet and saute until cabbage and onion are tender. Be sure cabbage browns a little for maximum flavor. Let cool before using.
To make dough: In a bowl, place flour and salt and mix well. Make a well in center of flour, add eggs, oil and water. Using your hand, mix until a sticky dough forms. If dough seems dry, add additional water 1 tablespoon at a time.
Lightly flour work surface and knead dough until smooth, about 5 minutes. Divide dough into 6 balls. Reflour board lightly and using a rolling pin, roll dough into a very thin strip, about 6 inches x 9 inches. At the edge of the 9 inch side, place spoonfuls of filling about 1 inch apart along entire edge. Pull the other 9 inch edge up and over the filling. Using your fingers, shape dough around the filling. Cut pierogis out with a pizza cutter and pinch edges to seal. Place on a baking sheet. Continue until all the dough and filling is used. At this point you may place in freezer, single layer, and freeze until firm. The place in freezer bags for later use.
If you are cooking them now, bring a large pasta pot full of salted water to a rolling boil. Add pierogis making sure not to overcrowd. When water come back to a full boil, watch as they need about 2 more minutes and they're done.
In a skillet, place butter and onion and over medium heat, allow to sweat until onion is transparent. We don't want them to fry, only cook gently. As pierogis finish boiling, drain and place immediately into skillet. Allow to cook for additional few minutes. Makes lots. Enjoy!