2 lb. russet potatoes, peeled and sliced 1/2″ thick
2 c. shredded sharp cheddar cheese
4 T. unsalted butter
2 – ½ c. (13 – ¾ oz.) bread flour
1 t. baking powder
1 c. sour cream
1 large egg plus 1 yolk
1 onion, chopped fine
4 T. unsalted butter
1/2 t. salt
1. FOR THE FILLING: Combine potatoes and 2 tablespoon salt in large saucepan and cover with water by 1 inch. Bring to boil over medium-high heat; reduce heat to medium and cook at vigorous simmer until potatoes are very tender.
2. Drain potatoes in colander. While still hot, combine potatoes, cheddar, butter, 1 teaspoon salt in bowl of stand mixer. Fit mixer with paddle and mix on medium speed until potatoes are smooth and all ingredients are fully combined, about 1 minute. Transfer filling to 8-inch square baking dish and refrigerate until fully chilled, about 30 minutes, or cover with plastic wrap and refrigerate for up to 24 hours.
3. FOR THE DOUGH: Whisk flour, baking powder, and 1/2 teaspoon salt together in clean bowl of stand mixer. Add sour cream and egg and yolk. Fit mixer with dough hook and knead on medium-high speed for 8 minutes (mixer may “buck” from thick dough). Transfer dough to floured bowl, cover with plastic, and let rest 30 minutes, or refrigerate until ready to assemble.
4. Line rimmed baking sheet with parchment paper and dust with flour. Roll dough on lightly floured counter into 18-inch circle, about 1/8 inch thick. Using 3-inch biscuit cutter, cut circles from dough. Place just under 1 tablespoon chilled filling in center of each dough round. Fold dough over filling to create half-moon shape and pinch edges firmly to seal. Transfer to prepared sheet.
5. Gather dough scraps and reroll to 1/8-inch thickness. Cut more circles from dough and repeat with remaining filling. Cover pierogi with plastic and refrigerate until ready to cook, up to 3 hours.
6. FOR THE TOPPING: Melt butter in 12-inch skillet over medium-low heat. Add onion and salt and cook until onion is caramelized, 15 to 20 minutes. Remove skillet from heat and set aside.
7. Bring water to boil in Dutch oven. Add 2 tablespoons salt and half of pierogi to boiling water and cook until tender, about 8-9 minutes. Using spider or slotted spoon, remove pierogi from water and transfer to skillet with caramelized onion (or just plain butter). Return water to boil, cook remaining pierogi, and transfer to skillet with first batch.
8. Add 2 tablespoons cooking water to pierogi in skillet. Cook over medium-low heat, stirring gently, until onion mixture is warmed through and adhered to pierogi. Transfer to platter and serve.
TO MAKE AHEAD: Uncooked pierogi can be frozen for several weeks. After sealing pierogi in step 4, freeze them on baking sheet, about 3 hours. Transfer frozen pierogi to zipper-lock freezer bag. When ready to cook, extend boiling time in step 7 to about 7 minutes.
Adapted from Cooks Country