Pierogi (Polish Dumplings)

FOR THE PIEROGI DOUGH: Pulse flour and salt together in a food processor until combined, about 4 pulses. With machine running, slowly add egg, egg yolk, and oil through feed tube until mixture resembles wet sand, about 30 seconds. With machine running, slowly add 4 tablespoons of water until dough forms a ball. If dough doesn’t ball up, add remaining water, 1 tablespoon at a time, with processor running until dough ball forms (you may not use all water). Dough should feel very soft and malleable.

Transfer dough to lightly floured work surface and knead by hand until it firms slightly and becomes smooth, about 2 minutes. Cover with plastic wrap and set aside to rest for at least 15 minutes or up to 2 hours.

FOR THE FILLING: Meanwhile, cover potatoes by 1 inch of water in large saucepan and add 1 tablespoon salt. Bring to boil, then reduce to simmer and cook until potatoes are tender and fork can be slipped easily into center, 10 to 12 minutes. Drain potatoes in fine-mesh strainer.

Set food mill or ricer over medium bowl and process potatoes into bowl. Stir in cheeses and butter until incorporated and season with salt and pepper to taste. Cool slightly before filling pierogi or cover with plastic wrap and refrigerate until needed, but no longer than 2 days.

FOR THE CARAMELIZED ONIONS: Melt butter in 12-inch nonstick skillet over medium heat. Add onions and salt and cook until very soft and well browned, 15 to 20 minutes. Measure out ¼ cup of onions, chop fine, and stir into filling. Cover pan of caramelized onions to keep warm.

FOR THE PIEROGI: Dust baking sheet liberally with flour; set aside. Divide dough into 2 even pieces and cover with plastic wrap. Working with 1 piece of dough at a time, unwrap dough and roll out on lightly floured counter into 15-inch circle, about 1/16 inch thick. Using 3-inch round cutter, cut out as many rounds as possible. Carefully gather up dough scraps, wrap in plastic wrap, and set aside. Place 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 and cover with damp dish towel. Repeat with remaining dough rounds.

Gently knead dough scraps together into ball and let rest for 5 to 10 minutes. Roll out, cut, and assemble additional pierogi, discarding any remaining dough scraps. (Towel-covered baking sheet of pierogi can be wrapped with plastic wrap and refrigerated for up to 4 hours. Pierogi can also be frozen for up to 1 month; when completely frozen, pierogi can be transferred to zipper-lock bag to save space in freezer. Do not thaw before boiling.)

Bring 4 quarts water to boil in large pot. Add 1 tablespoon salt and half of pierogi. Cook, stirring often, until edges feel al dente, 5 to 6 minutes (8 to 10 minutes if frozen). Using spider skimmer or slotted spoon, transfer pierogi to fine-mesh strainer and set aside. Return water to boil and cook remaining pierogi.

While second batch of pierogi is boiling, melt 2 tablespoons butter in 12-inch nonstick skillet over medium-high heat. Add first batch of boiled and drained pierogi and sauté until golden on both sides, 1 to 2 minutes per side. Transfer browned pierogi to platter and cover to keep warm. Drain and sauté remaining pierogi using remaining 2 tablespoons butter. Sprinkle remaining warm caramelized onions over top and serve with sour cream.