Preheat oven to 350°F. Line a sheet pan with parchment paper.
Place flour and yogurt in a medium bowl; stir well with a spoon until just combined and then continue to knead dough by hand in the bowl until smooth, about 1 to 2 minutes. If dough is too sticky to knead, add 1 to 2 more tablespoons flour; let dough sit 5 minutes.
Meanwhile, in a medium bowl, combine mashed potato, spinach, green onions, and 2 tablespoons yogurt until well-combined; season with salt and pepper.
Place dough on a work surface sprinkled with a little flour; roll dough into a rectangle. Spoon potato mixture in a log shape along one long edge of rectangle; brush opposite edge of dough with remaining 1 tbsp yogurt. Roll dough over filling and continue rolling up tightly, jelly-roll style.
Using a knife, make 6 evenly-spaced indentations along rolled-up dough. Using the side of your hand, press down on dough along indentations to separate dough into 6 pieces; seal ends of each piece and mold each into a circle. Lightly brush knish tops with a little yogurt mixed with a few drops of water.
Bake until lightly browned, about 25 minutes.
Serving size: 1 knish
Serving size: 1 knish
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