Prepare the filling:

1. Place potatoes in a large pot. Add water to cover the top of the potatoes by about an inch. Add 1 teaspoon of salt and stir well to dissolve. Bring it to a boil over medium-high heat and cook until potatoes fork tender, about 20 minutes. Insert a fork to check doneness.
2. Drain the water and use a potato masher or fork to mash the potatoes.
3. Add cheddar cheese, garlic powder, sour cream, remaining salt, and pepper. Stir well until evenly mixed and let it cool to room temperature or refrigerate until ready for use.

Make the pierogi wrapper:

1. In a large mixing bowl, whisk together flour and salt until evenly combined. Add water, sour cream and egg. Stir well with a spatula until no dry flour is visible. Make sure to scrape off any remaining flour on the sides of the bowl.
2. Transfer the dough onto a clean and dry surface. Knead the dough until it turns into a soft and smooth dough ball, about 10 minutes. You can also knead the dough on medium speed in a stand mixer for 10 minutes until smooth.
3. Cover the dough with plastic cling wrap and let it rest for 10 minutes.
4. Use a bench scraper or knife to divide the dough into 4 pieces. Lightly flour a clean surface and a rolling pin. Take one piece of dough at a time and use a rolling pin to roll it out to 1/16-inch thickness.
5. Use a 3-inch round cookie cutter to cut out circles from the dough. Repeat with the remaining dough balls. 
6. Combine the scraps into one dough ball and re-roll and repeat until all the dough is used up.

Assemble the perogies:

1. Prepare a small bowl of water for dipping your fingers into. Take one pierogi wrapper at a time and place it in your palm. Dip your finger into water and smear it along the edge of the wrapper.
2. Add a tablespoon of filling onto the centre of the wrapper. Fold the wrapper over into a half-moon shape and enclose the filling inside. Press the edges together to seal, or pleat the edges together with your finger. Place the pierogies on a parchment-lined baking tray.
3. Repeat with all wrappers. Cover with a tea towel to prevent the wrappers from drying out until ready to cook.

Make fried onions:

1. Melt butter in a large skillet over medium-high heat. Add diced onion and sauté over medium-high heat until the onions are caramelized and crispy, about 10 minutes. Season with salt to taste.

Cook the perogies:

1. Add perogies (in batches) to a large pot of boiling water and cook until they are floating, about 3-5 minutes. Cook for another 2-3 minutes until the dough becomes tender.
2. Serve warm with a dollop of sour cream, green onions, and fried onions on top.