4-4 3/4 cups flour
2 tsp baking powder
1 tsp salt
1 cup neutral oil, such as canola
2 tsp white vinegar
1 cup warm water
1/4 cup neutral oil, such as canola
2 lbs yellow onions, diced
5 lbs red potatoes, peeled and quartered
1 1/2 tsp salt
1/2 tsp black pepper
1 1/2 tsp garlic powder
2 Tbsp butter (optional)
mustard for serving
Mix 4 cups of the flour, baking powder, and salt in the bowl of a mixer. Beat together the eggs, oil, vinegar and warm water, and add to the dry ingredients. Mix with a dough hook for 8-10 minutes, adding additional flour as needed until the dough just clears the sides of the bowl (it will still be very soft). Divide the dough into two equal portions, form into balls, and let rest at room temperature, covered, for at least an hour.
While the dough is relaxing, prepare the filling. Heat the oil in a heavy skillet over a medium flame. Add the onions and saute until well caramelized, stirring every few minutes. You want them deeply browned, which will take ~30-40 minutes. Turn down the flame if this goes too quickly--lower heat=more even caramelization=more flavor.
While the onions are cooking, place the potatoes in a pot of water and bring to a boil. Lower the heat until it's just enough to maintain a simmer, and cook until the potatoes are fork-tender. Drain well, and mash well (or press through a ricer). Add the browned onions, salt, pepper and garlic powder, and mix well. Taste and adjust seasonings as needed, adding butter if desired. Let cool.
When the dough has relaxed and the filling has cooled, preheat your oven to 375 degrees and clean off a table or huge countertop. Take out one of the dough balls, and roll it into a rectangle as thin as possible--roughly 1 foot by 2 1/2 feet. If you'd like bigger knishes, you can make a wider rectangle. Some small holes are okay (the multiple layers of dough will cover them). If it's not rolling easily, let it relax a few minutes, and try again. Take half the potato filling, and evenly distribute it in a thick snaky line along the long edge, leaving about 1" of dough. Pull the edge of the dough over the filling, and roll until you get to the opposite edge, making a nice thick rope. You want the initial rolls to tuck the filling together neatly so that you don't have gaping airholes inside, but not ridiculously sushi-roll tight (otherwise you risk knish explosions in the oven).
Pinch the rope where you'd like to cut the first knish, slightly stretching the dough and drawing it together (I favor knishes that are a bit smaller than tennis balls). Twist it around a few times, creating a little twist of dough between the knish you're shaping and the main rope, and pinch off the knish. Repeat until all the knishes have been pinched off. If any of the pinched-off edges open, gently draw the dough over the top and pinch the ends together to re-seal. Press gently on the center, flattening it into a chubby puck, and creating a slight indentation in the center. Repeat to shape the remaining knishes.
Place knishes on a tray lined with parchment or silpat (or well-greased, if you don't have either of the former), and bake until lightly browned (~40 minutes, although time will vary depending upon knish size). If you would like to set some aside for the future, freeze them unbaked, on a lined tray, and then move to a bag or container. Bake as with fresh knishes, adding ~5 minutes to the baking time. Serve with mustard.