Pierogi

Make the Dough: In a large bowl, mix the flour and salt. Add the room temperature water into a liquid measuring cup that holds at least 2 cups of liquid. Add the eggs to the water and whisk with a fork until evenly combined. Add the water/egg mixture to the dry ingredients and stir with a fork to evenly moisten until the ingredients come together into a loose, shaggy mass. Using your hands, knead the dough in the bowl until it comes together into a ball. Lightly dust a clean work surface with flour. Invert the dough onto the surface and knead for 1 to 2 minutes, dusting with more flour as necessary, until smooth and elastic. Wrap the dough in plastic wrap and let rest at room temperature for 30 minutes.

Make the Filling: Melt the butter in a medium skillet over medium heat. Add the onions and season with ⅛ teaspoon salt and a few grinds of black pepper. Cook, stirring frequently, until golden brown and softened, 8 to 10 minutes. Remove from the heat. Set aside about 1 cup of onions for the garnish.

In a medium bowl, combine the onions from the skillet with the mashed potatoes, cottage cheese, 1 teaspoon salt, and ¼ teaspoon pepper. Mix well.

Roll and cut the dough: Divide the dough into two even pieces. (Wrap one piece back in plastic wrap while you work with the other piece.) Dust some flour onto a clean work surface and onto a baking sheet (that will hold the pierogi), then roll out one portion of dough as thin as possible, about 1/16 inch. Using a 3-inch cookie cutter or inverted glass, cut out as many disks of dough as you can. Remove the scraps and wrap them in plastic wrap (you'll re-roll them later).

Assemble the pierogi: Have a small bowl of water nearby to help moisten the dough edges if necessary. Working with one dough disk at a time, spoon a scant tablespoon of filling onto the center of the disk. Fold the dough in half over the filling, creating a crescent shape. To seal, pinch the top edges together firmly. Then continue sealing down both sides using a pinch-and-twist method: with your thumb and forefinger, pinch a small section of dough near the top, twist it slightly upward, and press it into the next section of dough. Repeat this pinch-and-twist motion all the way down both sides to create a secure, sealed, and very thin edge. Once sealed, place the pierogi on the prepared baking sheet. Repeat this process for the remaining dough disks and filling. Work fairly quickly, as the pierogi can be harder to seal if they begin to dry out. Moisten the inside of half the disk only if necessary to help the dough adhere.

Unwrap the remaining dough and repeat the process. You can re-roll the scraps one time by forming them into a ball and rolling them out again, but avoid rolling them out more than once, as this can make the dough tough.

Cook the pierogi: In a large pot, bring 3 quarts of salted water to a boil over high heat.

Add a single layer of pierogi to the boiling water, stirring gently to ensure they don't stick together. Let them cook for 4 to 5 minutes. Use a slotted spoon or spider strainer to transfer the cooked dumplings to a tray or baking sheet and toss with a light coating of oil to prevent sticking. Repeat with the remaining pierogi.

If you'd like to pan-fry the pierogi after boiling them, work in batches. Melt 1 to 2 tablespoons of butter in a large skillet over medium-high heat until it begins to crackle. Add the boiled pierogi in a single layer, ensuring they don't overlap, and cook until crisp and golden, 1 to 2 minutes per side. Repeat with the remaining pierogi, adding more butter as needed. Serve hot, topped with reserved onions and parsley or dill (if using), and with sour cream on the side.