Step 1: Mix and rest your dough

1. In a large bowl, combine the all-purpose flour, beaten egg, vegetable oil, salt, and 1 cup of warm water.
2. Mix everything together until a shaggy dough forms, then bring it together with your hands. If the dough feels stiff and won’t come together, add water a tablespoon at a time until it hydrates. You’re looking for a soft, slightly tacky dough, not a sticky one.
3. Turn the dough out onto a lightly floured surface and knead for 4 to 5 minutes, working it until the surface becomes smooth and pliable and springs back gently when you poke it.
4. Wrap the dough tightly in plastic wrap and let it rest at room temperature for at least 30 minutes and up to 1 hour. This rest period is non-negotiable: the gluten needs to relax before you roll it, or the dough will keep snapping back and fighting you the whole time.

Step 2: Make the filling

1. In a medium skillet over medium-low heat, melt 2 tablespoons of butter and add the finely diced white onion.
2. Cook slowly, stirring occasionally, until it turns soft and translucent without picking up any brown color, about 8 to 10 minutes. The goal here is tender and sweet, not caramelized.
3. Transfer the cooked onion to a large bowl with your mashed potatoes. Add the shredded cheddar cheese and the remaining butter, then stir the whole mixture together until the cheese melts fully and the potatoes become very smooth and uniform.
4. Taste the filling and season generously with salt and black pepper. A bland filling means a bland pierogi, so don’t be shy here.

Step 3: Set up your assembly station

1. Line two baking sheets with parchment paper before you start rolling. Once you’re in the rhythm of cutting and filling, you’ll want a clean surface to place finished pierogies without them touching each other.
2. Divide the dough in half and keep the rest of the dough covered in plastic wrap so it doesn’t dry out while you work the first half.

Step 4: Roll and cut the dough

1. On a lightly floured surface, roll out the first half of your dough to about one-eighth of an inch thick. It should feel smooth and cooperative after its rest.
2. Use your 3-inch round cookie cutter to cut circles from the dough as efficiently as possible.
3. Gather the scraps, press them together, and let them rest for a few minutes before re-rolling to cut more circles. You’ll get a couple of extra rounds from each batch of scraps, so don’t toss them.

Step 5: Fill and seal

1. Scoop about one tablespoon of the filling and roll it into a smooth ball between your palms. Place the ball in the center of a dough circle.
2. Fold the dough over the filling to form a semi-circle, pressing out any air pockets as you press the edges together firmly with your fingers.
3. Crimp the entire edge with the tines of a fork to create a secure seal. Press firmly enough to create a visible impression but not so hard that you cut through the dough.
4. Place each finished pierogi in a single layer on your lined baking sheet.
5. Continue with the remaining filling and the rest of the dough, keeping finished pierogies covered with a clean kitchen towel to prevent drying out.

Step 6: Boil the pierogies

1. Bring a large pot of generously salted water to a rolling boil.
2. Working in batches, gently lower the pierogies into the boiling water without crowding them. They need room to move so they don’t stick together.
3. Cook until they float to the surface, about 2 to 4 minutes per batch.
4. Once they float, give them an additional 30 seconds to ensure the dough is fully cooked through, then remove them with a slotted spoon and transfer to a plate. They’ll look pillowy and slightly translucent at this stage. That’s exactly right.

Step 7: Pan-fry to golden brown

1. In a large skillet over medium heat, melt 3 tablespoons of butter until it foams.
2. Add your boiled pierogies in a single layer and cook for about 2 to 3 minutes per side, until each one develops a golden brown, slightly crisp surface.
3. Work in batches if needed, adding more butter between rounds.
4. While the last batch of pierogies finishes, add your second diced or sliced onion to the same pan with a little extra butter if needed.
5. Cook over medium-low heat until soft and golden, about 8 to 10 minutes. These serving onions are worth every extra minute on the stove.
6. Serve the pierogies warm, topped with those onions and a cold dollop of sour cream on the side.