1 large egg
1 tablespoon Kosher salt
1 tablespoon sour cream
1 1/4 cup water
4 cups flour
POTATO FILLING :
1 pound potatoes (peeled)
4 slices American cheese
1 tablespoon salt
CARAMELIZED ONION AND MUSHROOM TOPPING:
5 tablespoons unsalted butter (divided)
3 medium yellow onions (sliced thinly)
2 tablespoons olive oil
2 cups cremini mushrooms (thinly sliced)
1/4 cup blue cheese (crumbled)
3 tablespoons chives (chopped, to garnish)
Kosher salt (to taste)
For the Dough: In a large bowl, whisk together the egg, salt, sour cream and water. Slowly add the flour, one cup at a time, and mix with a wooden spoon, kneading by hand when necessary to incorporate all the flour. The dough is ready when it is no longer sticky. Set the dough on a lightly floured surface, cover with plastic wrap until ready to use.
For the Potato Filling: Place the peeled potatoes into a large pot and cover them with water. Bring to a boil and cook until tender, about 30 minutes. Drain the potatoes and process them through a potato ricer or in a large bowl, mash them until they are a smooth. Add American cheese and salt to the potato mixer, stirring with a wooden spoon to combine. Set aside.
To Assemble: Split the dough in half, reserving the remaining dough.
On a lightly floured work surface, roll out dough to an 1/8-inch thickness and cut into 3-inch rounds. Spoon 1 tablespoon of filling onto each round, being careful not to overstuff. Fold the dough over into half-moons and press the edges with a fork to seal them.
Bring a salted pot of water to a boil. In batches, add the pierogies and cook until they begin to float. Cook for 4 minutes longer and drain, reserving 1 cup of the cooking liquid.
In another large saute pan add olive oil and heat over medium-high heat. Add mushrooms and cook until tender, about 7 minutes.
Add remaining 2 tablespoons of butter to the large saute pan over medium heat. Add the pierogies and sear until golden on both sides, about 3 minutes per side. Add the caramelized onions and toss to combine. Garnish with blue cheese and chives.
For the Caramelized Onion Topping: In a large saute pan, heat 3 tablespoons butter over medium heat. Add the onions and arrange in one even layer. Cook the onions, stirring occasionally to ensure even cooking, until onions caramelize, about 25 minutes. Season with salt. Set aside.
In the same saute pan add the olive oil and heat over medium heat. Add the mushrooms and cook for 7 minutes or until tender.
Tip: Form and freeze the pierogies in advance. Store on a baking sheet wrapped with plastic until ready to serve.