In a bowl, beat egg, soft butter & sour cream together. Add flour & salt, combining until smooth. Scrape dough out of bowl onto a lightly floured work surface & knead dough for 5 – 8 minutes. Wrap dough in plastic wrap & refrigerate until ready to use.
While you are cooking the onions & mushrooms, peel & quarter the potatoes & place in a small pot. Cover with cold water, bring to a boil, then lower heat & simmer until soft enough to mash, 15-20 minutes. Drain & place in a large mixing bowl.
Mash & whip potatoes with grated cheddar cheese until smooth & fluffy. Fold in the cooked mushrooms & caramelized onions until well blended. Salt & pepper to taste.
Assembly
Remove perogy dough from refrigerator & cut into 24 equal pieces. Roll each piece into about a 3-inch round. Place a heaping Tbsp of filling in the middle of pierogi. Dip your finger in a bowl of water & run it along the edge of the dough. Fold perogy in half, carefully pinching together edges to seal it completely.
Cook & Serve
Bring a large pot of salted water to a boil. Carefully drop perogies in & boil until all the perogies float to the surface & dough becomes somewhat translucent, about 3-5 minutes. Remove perogies with a slotted spoon, making sure to let as much of the excess water drip off as possible.
In a large skillet, heat a Tbsp of butter. Place drained pierogis in skillet. Do not over-crowd so that they can all lightly brown on both sides.
If you wish , you can make a few extra onions & mushrooms for a garnish. Serve with either sour cream or Ranch dressing.
Recipe Notes
When making the pierogis, nothing wrong with rolling out all the dough at the same time & cutting your circles with a cookie cutter. I just personally like dividing the dough so I don’t have to do any re-rolling with the scraps. Just personal preference.