Place the flour and salt into the body of a stand mixer with the hook attachment. Stir to combine.

Add the eggs and mix them in. The mixture will appear crumbly.

Stir in the sour cream and softened butter. Slowly stream in the warm water.

Once the dough is combined, turn the mixer on medium-high speed and knead for 7 minutes until a smooth, soft dough forms. Spray lightly with cooking spray and roll the dough in the spray so it is coated. Cover with plastic wrap and let it rest for 30 minutes.

While the dough rests, make the filling. Place the diced potatoes in a medium-sized pot covered 2 inches above the potatoes with cold water.

Bring to a boil, reduce heat to a simmer, and simmer until the potatoes are fork tender, 12-15 minutes.

Drain the potatoes well. Place them in a large bowl and mash until smooth.

Stir in the butter, salt, and pepper until melted. Add the cheese and stir to combine. Set aside.

Divide the dough into two portions, and keep the second one covered while working with the first.

Place the dough on a lightly floured, clean work surface and roll it out to about 1/8th inch thick. Take a 3-inch or similar-sized circle cookie or biscuit cutter and cut out circles from the dough.

Take about 1-1/2 teaspoons of the potato filling and place it in the center of the circle. Using your fingers, pinch around the edges to form a half-moon shape. Make sure to pinch firmly to secure the edges.

Place on a parchment-lined sheet tray while you make the remaining pierogi. Cover them with a clean kitchen towel so they don’t dry out.

Boil a large stock pot of water and season it generously with salt.

While the water comes to a boil, add the butter, onion, salt, and pepper to a large skillet over medium-low heat. Stir occasionally and cook the onions until they start to brown, 12-15 minutes.

Place half of the pierogies in the boiling water. Boil until they float, 3-4 minutes, stirring occasionally.

Using a slotted spoon, take the pierogies out of the boiling water and immediately place them into the skillet with the onions.

Gently stir the pierogies to coat them in the butter and onions, and cook until lightly toasted on the outside, about 4 minutes. Toss them frequently so they toast on both sides.

Serve with sour cream.