For the dough: Put the flour in the bowl of a food processor. Pulse as you pour the oil through the feed tube. Scrape down the sides of the processor and pulse again.
Combing the seltzer and vinegar in a measuring cup. Add 1 teaspoon salt and stir to dissolve. Pulse as you pour the liquid through the feed tube into the flour mixture, 20 to 30 seconds. Turn the dough out onto a floured surface and knead until smooth, about 1 minute. Flatten into a disk, wrap in plastic wrap and refrigerate at least 2 hours.
For the filling: Put the potatoes in a saucepan, cover with water and season the water liberally with salt. Bring to a boil over medium-high heat, then reduce to a simmer and cook until tender and a fork inserted in the center goes in without any resistance, 35 to 40 minutes. Drain and cool slightly. (You can put the potatoes in the freezer for a few minutes to hurry along the cooling process.)
Meanwhile, place the spinach in a clean dishtowel, wrap tightly and wring out as much liquid as possible. Roughly chop the spinach and reserve.
Heat 3 tablespoons of the oil in a large skillet over medium heat. Add the onions, 1/4 teaspoon salt and 1/4 teaspoon pepper and cook, stirring occasionally, until browned, about 15 minutes. Add the garlic and cook until softened, about 2 minutes. Add the spinach, 1/2 teaspoon salt and 1/4 teaspoon pepper and stir to combine. Turn off the heat and reserve.
Remove the dough from the refrigerator and let sit at room temperature just until it is pliable, 30 to 45 minutes.
When the potatoes are cool enough to handle, use the side of a spoon to peel away the skins. Transfer the potatoes to a large bowl. Add the cream and remaining tablespoon of oil and coarsely mash. Add the spinach mixture to the mashed potatoes and gently stir to combine. Season to taste with salt and pepper.
Adjust an oven rack to the middle position. Preheat the oven to 375 degrees F. Lightly brush a baking sheet with oil.
To make the knishes: Put the dough on a floured sheet of parchment and dust the top with flour. Stretch the dough into a 6-by-8-inch rectangle, then use a rolling pin to roll it into a 12-by-18-inch rectangle. Arrange the dough with one of the long sides of the rectangle facing you. Put the filling along the length of the dough and shape it into a compact 14-by-2-inch log, leaving a 2-inch border of dough on the side closest to you and on each end. Use the parchment paper to help roll the dough around the filling, brushing off the flour as you roll and stretching the dough as needed to make a tight, dough-covered log.
Pinch the dough closed along the center seam, then carefully turn the roll seam-side down on the parchment. Pull on the dough at each end of the log to twist and seal it closed; pinch off the excess dough. Using the handle of a wooden spoon, divide the log into 3 even sections, pressing down through the log to seal the segments. (The log will resemble three links of sausage.) With a sharp knife, cut through the connections and separate the links. Pinch the ends to ensure that each seal is secure. Cut each of the links in half crosswise to make 6 knishes total. Arrange the knishes cut-side up, pressing to flatten each into a disk about 1 1/2-inches thick.
Arrange the knishes on the prepared baking sheet 2 inches apart. Bake 30 minutes; liberally brush the knishes all over with oil and continue baking until golden, 25 to 35 minutes more. Serve with mustard.
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