Spinach Potato Knish

Name: Spinach Potato Knish
Ingredients: 1 1/2 cups all-purpose flour, plus more for dusting; 3 tablespoons extra-virgin olive oil, plus more for brushing; 1/4 cup plus 2 tablespoons cold seltzer; 2 teaspoons apple cider vinegar; Kosher salt and freshly ground black pepper; 1 1/2 pounds medium russet potatoes (about 3); One 16-ounce bag frozen whole or cut-leaf spinach, thawed; 4 tablespoons extra-virgin olive oil, plus more for brushing; 2 medium yellow onions, small dice (about 2 1/2 cups); 4 cloves garlic, minced; 1/4 cup heavy cream, warmed; Spicy mustard, for serving