1. Make the dough: In a large bowl, combine the flour and salt. Make a well in the center. In a separate bowl, whisk the hot water, egg, and sour cream until smooth. Pour the wet mixture into the flour well. Use a fork to stir from the center outward until a shaggy dough forms.

2. Turn the dough out onto a floured surface. Knead for 5-7 minutes until smooth and elastic. Shape into a ball, place back in the bowl, cover with a damp towel, and let rest for 30 minutes.

3. Make the filling: While the dough rests, place the potatoes in a pot, cover with cold salted water by an inch, and bring to a boil. Simmer for 15-20 minutes until fork-tender.

4. Meanwhile, melt half the butter (2 tablespoons) in a skillet over medium heat. Add the diced onion and cook for 4-5 minutes until softening. Reduce heat to low-medium and cook for another 10-12 minutes, stirring occasionally, until deeply golden. Remove half for the filling and set the rest aside for serving.

5. Drain the cooked potatoes well. Return them to the hot, dry pot for a minute to steam off excess moisture. Mash until smooth. Immediately stir in the grated cheddar, the reserved half of the caramelized onions, salt, and pepper. Set aside.

6. Assemble the pierogi: Divide the rested dough in half. Keep one half covered. On a floured surface, roll the other half out to about ⅛-inch thickness. Use a 3-inch round cutter to cut out circles. Re-roll scraps once.

7. Place a heaped teaspoon of filling in the center of each circle. Brush the edges with a little water. Fold the dough over to create a half-moon, pressing edges firmly to seal. Press with a fork around the curved edge for a tight seal. Place on a parchment-lined baking sheet. Repeat with second half of dough.

8. Cook: Bring a large pot of salted water to a boil. Working in batches, gently drop pierogi into the water. They will sink at first. Once they float to the top, boil for an additional 2-3 minutes. Remove with a slotted spoon.

9. To serve: Melt the remaining 2 tablespoons of butter in a large skillet over medium heat. Pan-fry the boiled pierogi for 2-3 minutes per side until golden and crispy. Serve immediately with reserved caramelized onions and sour cream.