Knyshi with potatoes and caramelized onions

Ingredients:

Dough

• 2.5 cups premium flour + extra for working with the dough
• 1 teaspoon baking powder
• 1 teaspoon coarse salt
• 6 tbsp schmaltz (melt and cool) or neutral vegetable oil*
• 1 teaspoon distilled white vinegar

Filling

• 2 tbsp. schmaltz or neutral vegetable oil
• 2 large yellow onions, diced into medium cubes
• 1 tbsp chopped thyme leaves (about 12 sprigs)
• 2 cloves garlic, crushed
• 700 g Russet-Burbank potatoes, cut into medium pieces
• 140 g of cream cheese, room temperature
• 1/3 cup sour cream + extra for serving
• 1/4 cup chopped dill
• 2 tbsp chopped chives (4-6 shoots)
• 1 large egg

Preparation:

1. Dough:
   
   In the bowl of a stand mixer, combine the flour, baking powder, and salt. Add the schmaltz, vinegar, and 0.5 cups of water. Mix with a spoon or large silicone spatula. It's okay if some of the flour remains dry at the bottom of the bowl; the dough will come together as you mix. Attach the dough hook to the stand mixer and mix on medium speed until the dough is smooth and elastic, about 8 minutes.
2. Turn the dough out onto a lightly floured surface and knead it into a smooth ball, about 30 seconds. Wrap the dough in plastic wrap and refrigerate for at least 30 minutes while you prepare the filling. The dough can be made a day ahead and refrigerated.
3. Filling:
   
   Heat the schmaltz in a heavy-bottomed saucepan over medium heat. Add the onion and 2 tablespoons of salt and stir. Cover and cook the onion, stirring occasionally, until translucent and beginning to darken around the edges, about 15 minutes.
   
   Note *
   
   Schmaltz is chicken fat. Look for it in the meat section of the grocery store.
4. Continue cooking, uncovered, stirring occasionally at first, then more frequently as the onions become sticky, soft, and deep golden, about 30 minutes. Add the thyme and garlic and cook, stirring and scraping up any browned bits, until fragrant, about 1 minute. Remove from heat and set aside.
5. Cover the potatoes in a medium saucepan with cold water. Bring to a boil, then reduce heat and simmer until tender, 15-20 minutes.
6. Drain and return the potatoes to the pan. Heat the potatoes over low heat, shaking the pan to dry them out and prevent them from sticking to the bottom, for about 1 minute. In a large bowl, combine the potatoes, caramelized onions, cream cheese, sour cream, dill, and chives. Mash with a potato masher, distributing the ingredients evenly. Season generously with salt and pepper. Set aside to cool to room temperature. The filling can be made a day ahead, placed in a container, tightly sealed, and refrigerated.
7. Preheat oven to 350°F (175°C). Line 2 baking sheets with parchment paper or lightly spray with cooking spray and set aside. Whisk the egg with 1 tablespoon of water in a small bowl and set aside.
   
   
   When measuring flour, spoon it into a dry measuring cup and tip out any excess. Scooping flour directly from the bag with a measuring cup will compact it, resulting in dry baked goods.
8. Divide the dough in half. On a lightly floured work surface, use your hands to form one half of the dough into a 15 cm (6 in) log. Then, using a rolling pin, roll it out into a 40 x 30 cm (16 x 12 in) rectangle. The dough will be very thin. Trim the sides if necessary to prevent the edges from becoming too thick and uneven.
9. Place the dough so the long sides are parallel to the edge of the table. Form half of the filling into a log measuring 40 x 5 cm. Transfer the filling to the dough, 2.5 cm from the bottom edge. Gently stretch the dough over and around the filling. It's okay if the dough tears a little at this point; you'll be rolling it into a roll. Then continue rolling the filling in the dough. Make slits in the roll at 5 cm intervals, then cut it into 8 segments.
10. Working with one section at a time, turn the knysh so that the cut side is facing up, pinch and smooth the top to seal. Flip it over so that the other cut side is facing up, pinch and smooth that side to seal as well. Place the resulting knysh, sealed side down, on one of the prepared baking sheets, then flatten it lightly with your hand. Repeat with the remaining roll sections, dough, and filling to form all the knysh. At this point, they can be frozen and stored in the freezer, and baked as needed.
    
    

    Freezing Advice

    
    The knishes can be formed and frozen without baking for up to 1 month, arranged in a single layer in an airtight container. To bake the frozen buns, brush them with beaten egg and increase the baking time to approximately 1 hour 15 minutes.
11. Place the knyshes on baking sheets, spacing them about 2.5 cm apart. Lightly brush the tops and sides with the remaining beaten egg. Bake, rotating the baking sheets halfway through, until golden brown, 45-50 minutes. Let cool for 15 minutes and serve. Knyshes can be served hot, warm, or at room temperature with sour cream.