Saute onions in a generous amount of butter until soft, not brown.
Boil the potatoes, drain and cut up. Put potatoes back in warm pot and add the cheese. Put the lid on the pot so the cheese melts. Add onions to the potato mixture. Add salt and mash all the ingredients. Let cool slightly.
Mix the dough until soft and easy to roll thinly. It's stretchy. Keep the dough covered lightly with a towel to prevent drying.
Roll out a small amount of dough. Roll the dough into a 2-inch square or cut into that shape. Make sure you roll it thin. "After making a few, you'll get the hang of it."
Put about a rounded teaspoon of the mashed potato mixture on the dough square and bring over one point to the other to form a triangle. Seal edges. Continue this process until dough is used up. Lay pierogies on a clean cloth to dry. Turn the pierogies over occasionally as they dry.
After drying, boil a few at a time in a big pot of water, stirring constantly until they float. Spoon out pierogies into a colander in the sink and pour cold water over them to prevent sticking. After all the pierogies are boiled and rinsed, place in a casserole dish with lots of melted butter. Refrigerate until needed or freeze.
To serve, heat pierogies in a covered casserole until hot. Periodically, shake the casserole to loosen pierogies so they don't stick together. Serve with sour cream.