For the dough
2 cups all-purpose flour, plus more for dusting
3 large eggs
2 tablespoons sour cream
For the filling
1-1/2 lbs baking potatoes, peeled and cut into 1-inch slices
2 tablespoons unsalted butter
1 tablespoon extra-virgin olive oil
1/8 cup finely chopped sweet onion
1 small clove garlic, minced
1/4 teaspoon dried thyme
1/2 cup grated dry farmer's cheese
Kosher salt and freshly ground pepper to taste
To finish the dish
1/2 stick butter, cut into 1 tablespoon portions
To Prepare the dough
In a large bowl or mixer, combine the flour, eggs, sour cream and 1/2 cup of water. Beat the eggs as you mix and gradually add the rest of the water until the mixture is combined.
Turn the dough onto a well floured surface. Knead it gently, using a dropping technique (lift the dough from the surface and drop it down). Knead only until the ingredients are blended and the dough is smooth and slightly sticky, about 3 to 5 minutes. Be careful not to over work the dough.
Wrap the dough ball in plastic wrap and let it rest for at least 20 to 30 minutes while you prepare the filling.
To prepare the filling
Put the potatoes in a medium pot and add just enough cold, salted water to cover them. Bring to a boil and cook until the potatoes are soft, about 20 minutes.
While the potatoes cook, melt the butter and oil over medium heat in a large saute pan. Add the onion, garlic and thyme, cooking until the onion turns translucent, about 2 minutes. Lower the heat and continue cooking until onions caramelize slightly, about 20 minutes. You may need to add a bit more butter as the onion and garlic mixture cooks. Season with salt and pepper to taste and set aside to cool.
When the potatoes are soft, drain them in a colander and lightly press out the remaining moisture. Return them to the pot, remove from heat and add the cooled onion mixture and the cheese. Mash them just until blended and large lumps are gone. Season again with salt and pepper to taste. Set aside to cool while you roll out the pierogi dough.
Assemble and cook
Bring a large pot of salted water to a boil over high heat. With lightly floured hands, pinch off tablespoon sized portions of the dough and roll them into balls. The balls should be about 1-1/2 inches in diameter, yielding about 3 dozen total.
On a well floured surface, gently roll each ball with a rolling pin until about 1/8 inch thick and 3-1/2 inches round. Cover the finished rounds with a damp towel so they don't dry out while you're working.
Once your rounds are rolled out, hold each in the palm of your hand, filling the center of it with a generous tablespoon of the potato mixture. Gently fold the round in half, pulling the edges away and pinching them firmly shut to enclose the filling. Be sure the edges are sealed by working from one end to the other.
As you work, set your filled pierogi aside on a floured surface and cover them with plastic wrap.
Working in batches, drop no more than 6 pierogi at a time into the boiling water. After they float back to the surface, allow them to cook another 2 to 4 minutes. Remove the pierogi with a slotted spoon and place on a towel to drain and cool.
Sauté and serve
To finish the dish, heat a tablespoon or two of butter in a pan over medium heat and briefly sauté them in batches until they are slightly crispy and brown on the exterior.
