Pierogi Ruskies - Polish Dumplings

Bahan-bahan (30 porsi)

• 2 cups flour (, plain/all-purpose)
• 1 tsp cooking/kosher salt
• 50 g / 3 tbsp unsalted butter
• 1/2 cup water
• 1 large egg (, whisked (55-60g/2oz))
• 500 g / 1 lb potatoes (, peeled and sliced 1cm / 1/2" thick (2 medium, Note 1))
• 1 tbsp cooking/kosher salt ( - for cooking potatoes)
• 30 g / 2 tbsp unsalted butter
• 1 cup shredded cheddar cheese (, tightly packed cup (Note 2))
• 1/4 tsp cooking/kosher salt
• 1/2 tsp black pepper
• 30 g / 2 tbsp unsalted butter
• 1 onion (, finely diced)
• 1/2 tsp cooking/kosher salt
• 1 tbsp cooking/kosher salt (- for boiling water)
• 25 g / 1 1/2 tbsp unsalted butter (, per 8 - 10 pierogis (Note 3))
• Sour cream (, for serving)
• Parsley or chives (, finely chopped - just a pinch)