Make the dough: Put the flour in a food processor, then pulse as you pour the olive oil through the feed tube. Scrape down the sides of the processor and pulse again.
Dissolve 1 teaspoon salt in the seltzer and vinegar in a liquid measuring cup. Pulse as you pour the liquid through the feed tube, 20 to 30 seconds. Turn the dough out onto a floured surface and knead until smooth. Flatten into an even disk, wrap in plastic wrap and refrigerate at least 2 hours.
Make the filling: Put the potatoes in a saucepan, cover with water and season with salt. Simmer until tender, 20 to 30 minutes. Drain and coarsely mash in a large bowl.
Heat 2 tablespoons olive oil in a large skillet over medium-high heat. Add the onions, garlic and 1 teaspoon salt. Cover and cook about 5 minutes; uncover and cook, stirring, until browned, 15 to 20 more minutes.
Meanwhile, combine the remaining 2 tablespoons olive oil and the kasha in a small saucepan over medium-high heat and cook until toasted, about 4 minutes. Add 1 3/4 cups water and 1 teaspoon salt. Reduce the heat to medium, cover and cook until the water is absorbed, about 10 minutes. Season with salt and pepper. Cool slightly, then add to the bowl with the potatoes. Add the onions and mash.
Remove the dough from the refrigerator 30 minutes before making the knishes. Preheat the oven to 375 degrees F; lightly brush a baking sheet with olive oil.
Make the knishes; place 2 inches apart on the baking sheet. Bake 30 minutes; brush with olive oil and continue baking until golden, 25 to 35 more minutes. Serve with mustard.