This makes approximately 45 pierogies. You can use them right away, preparing them to serve with the instructions below. Or make them ahead of time and freeze all or a portion of them to use whenever you want a quick meal.
In a large mixing bowl, mix flour and salt.
In a separate bowl, beat together egg, water and oil. Slowly stir into flour mixture to make soft, not sticky, dough that holds together in a ball. If needed, add more water 1 tbsp at a time, being careful not to make the dough sticky.
Prepare a lightly floured surface, and kneed dough until just smooth (about 10 times). Split dough in half, cover with plastic wrap or damp cloth. Let rest in the refrigerator for 20 minutes.
In a pan, heat oil over medium heat. Cook onions for about 5 minutes, until soft and translucent.
Transfer to bowl and stir in mashed potatoes, cheese, salt and pepper.
Work with 1/4 of the dough at a time, keeping the remaining dough covered so it does not dry out.
Roll out dough on a lightly floured surface until about 1/16 of an inch thick. Using a 3-inch round cookie cutter, cut dough into rounds.
Note: Let left over dough rest before re-rolling.
Place 1 tbsp of filling into the center of each round. Slightly moisten, with water, the edge of half of each round. Fold each in half, pinch closed to seal, and crimp edges. Place on plastic wrap lined baking sheets, and cover with a damp cloth to prevent the pierogies from drying out.
You can use immediately or refrigerate for up to 8 hours. To freeze, freeze the pierogies on the baking sheets and then transfer to a freezer bag. Keep for up to one month and do not thaw before cooking.
Add 2 tbsp butter to pan and heat over medium high.
Add onion and red pepper to pan and cook for about 5 minutes until onions begin to soften, stirring occasionally.
Add beer and salt and pepper to taste. Bring to a boil, reduce heat and simmer, stirring occasionally, for about 15 minutes until onions are caramelized.
Remove vegetables from pan and set aside.
Bring a large pot of water to a boil.
Add the pierogies, and cook until floating, approximately 5-7 minutes.
Add the remaining tbsp of butter to the pan, and place over medium high heat.
Transfer boiled pierogies to the pan using a slotted spoon and fry until golden.
Serve pierogies with the caramelized vegetables and top with chopped green onion.
In a small bowl, mix together sour cream and sriracha. Serve on the side, or spoon on top of pierogies.