Add the water, oil, egg, and vinegar to the bowl of a stand mixer fitted with a dough hook and mix on medium speed until combine, about 3 minutes.

Add the flour, baking powder, and salt, and turn the mixer to low. Mix until everything is incorporated, about 3 minutes. Turn the mixer speed up to 2, and mix for 10-12 minutes. The dough should be silky and soft but not particularly sticky. If it is too sticky, add flour one tablespoon at a time as needed, and knead until a smooth dough forms, about 15 minutes. Wrap in plastic and let rest for an hour at room temperature before using.

In a medium sized pot, bring 1½ cups water, 1 tablespoon oil, and ¾ teaspoon of salt to a boil. Add the kasha groats, cover, and simmer for about 10 minutes until there the water is absorbed. Set aside, leaving the pot covered until it has fully cooled down.

Place the potatoes in a medium sized pot with 1 teaspoon of salt and cover with water by at least 2-inches. Bring to a boil over high heat, then reduce to a simmer and cook until the potatoes are fork tender, about 15-20 minutes. Once cool enough to handle, mash with a fork.

In a sauté pan, melt 2 tablespoons of butter with 2 tablespoons of olive oil over medium-low heat. Add the onion and sauté, stirring consistently to make sure they don't brown, about 15 minutes.

Add the kasha and onions to the pot with the mashed potatoes, then add 2-3 tablespoons of butter, 1 teaspoon of salt, and several grinds of black pepper. Mix well to combine, taste and adjust seasoning as needed. Let cool in the fridge before shaping the knishes.

Preheat the oven to 375.

Dust a clean work surface with flour. Cut your dough in half, re-wrap one half, and set aside until needed. Pat the dough flat into a rough rectangular shape and roll out with a rolling pin until thin and even (about 20-inches wide and 10-inches high).

Place about half of the kasha filling into a thick line about 1-inch from the bottom wide border of the dough. Brush the other long end of the dough with egg wash. Beginning with the bottom of your pastry, roll the dough upwards, keeping it it neat as you roll. You should be able to go around your filling twice. Avoid rolling it too tight as it will make the shaping of the individual knishes difficult.

Trim and seal the ends of the dough roll. Make marks at 3-inch intervals along the roll — each half of the dough should yield 6 knishes. Using the side of your hand, gently flatten the dough at each mark, then uisng a bench scraper or a knife, cut the dough. Cut the dough straight down at each mark to separate the pieces. Seal one end of each piece by pressing the dough firmly together. Place each knish upright on the sheet tray with the sealed end down, and gently press down until about 1½ inches tall. Repeat with the remaining pieces and the second piece of dough.

Brush each knish with egg wash and sprinkle with flaky salt. Bake until golden, about 35-40 minutes, rotating at the 20 minute mark. Serve with mustard. Store knishes in an airtight container for up to 3-4 days in the refrigerator.