Potato-Kasha Knishes

Name: Potato-Kasha Knishes
Prep time: 45 min
Cook time: 1 h 45 min
Total time: 2 h 30 min
Cuisine: Israeli
Ingredients: 1 1/2 cups all-purpose flour, plus more for dusting; 3 tablespoons extra-virgin olive oil, plus more for brushing; Kosher salt; 1/4 cup plus 2 tablespoons cold seltzer; 2 teaspoons apple cider vinegar; 1 1/2 pounds russet potatoes, peeled and quartered; Kosher salt; 4 tablespoons extra-virgin olive oil; 1 1/2 pounds Spanish onions, diced; 1 clove garlic, smashed; 1 cup whole or coarse kasha (roasted buckwheat groats; available in the bulk aisle or near the dried beans); Freshly ground pepper; Spicy mustard, for serving