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1
Prepare the Dough In a small bowl, stir the yeast and 1/2 teaspoon of the sugar into the warm milk.
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2
Let stand until foamy, about 5 minutes.
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3
Prepare the Dough In a large bowl, whisk the 1 1/2 cups of all-purpose flour with the buckwheat flour, salt and the remaining 1 tablespoon of sugar.
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4
Stir in the yeast mixture, beaten egg and melted butter and knead until a smooth dough forms.
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5
Transfer the dough to a lightly oiled bowl, cover with plastic wrap and let stand in a warm place until doubled in volume, about 1 1/4 hours.
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6
Punch down the dough, cover and let rise again until doubled, about 1 1/4 hours longer.
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7
Meanwhile, Prepare the Filling In a medium saucepan, cover the potato chunks with water and bring to a boil.
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8
Add a large pinch of salt and cook over moderately high heat until tender, about 8 minutes.
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9
Drain the potato and return the chunks to the pan, shaking over the heat to dry them out, about 10 seconds.
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10
Pass the warm potatoes through a ricer into a large bowl or mash them with a potato masher.
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11
Meanwhile, Prepare the Filling In a medium skillet, heat the 2 tablespoons of oil.
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12
Add the leek and a pinch of salt and cook over moderate heat, stirring occasionally, until softened, about 6 minutes.
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13
Remove from the heat and stir in the butter.
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14
Add the leek to the potatoes along with the cream cheese and heavy cream, stir well and season with salt and pepper.
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15
Meanwhile, Prepare the Filling Punch down the dough again.
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16
Transfer to a lightly floured work surface and shape it into a rectangle.
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17
Roll the dough out into a 8-by-20-inch rectangle, about 1/8 inch thick.
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18
Cut the dough into eight 4-by-5-inch rectangles.
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19
Set 1 rectangle with a long side toward you and spoon 1/4 cup of the potato filling on the top third; leave a 1/3-inch border on the top and sides.
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20
Bring the bottom of the dough up and over the filling, fold all the sides over themselves and press to seal.
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21
Repeat to form the remaining knishes.
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22
Brush the knishes with the beaten egg, coating the seams well.
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23
Place the knishes on a parchment paperlined baking sheet and freeze for 10 minutes.
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24
Meanwhile, Prepare the Filling Preheat the oven to 350.
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25
In a medium saucepan, heat 1 1/2 inches of vegetable oil to 375.
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26
Set a wire rack over a rimmed baking sheet near the stove.
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27
Fry 2 knishes at a time, turning them a few times, until richly browned, about 2 minutes.
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28
Using a slotted spoon, transfer the knishes to the rack to drain.
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29
Keep the fried knishes hot in the oven.
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30
Meanwhile, Prepare the Filling Cut the knishes on the diagonal into 2 1/2-inch pieces.
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31
Top with sour cream, dill, red onion and caviar and serve right away.
