Instructions

1. Whisk together flour and 1 tsp. salt in a medium bowl. Make a well in center and add 1 cup plus 2 tbsp. warm water and 2 tbsp. oil. Stir to form a dough and knead briefly. Divide dough into 4 pieces. Wrap in plastic and let rest for 30 minutes.
2. Meanwhile, put potatoes into a 4-qt. saucepan and add water to cover by 1". Season with salt, bring to a boil, and reduce heat to medium-low; simmer until tender, about 15 minutes. Drain potatoes and pass through a food mill or potato ricer into a large bowl. Heat remaining oil in a 12" skillet over medium heat. Add onions, partially cover, and cook, stirring occasionally, until soft, about 15 minutes. Whip onions into potatoes and season with salt and pepper.
3. Heat oven to 375°. Working with one piece of dough at a time, put dough on a lightly floured work surface and roll to 1⁄16" thickness. Using a 3" cookie cutter, cut out 16 circles. Spoon 1 tbsp. of potato mixture onto center of each circle. Brush edges with egg wash and bring up edges with your fingers and seal to form a purse. Transfer knishes to 2 greased baking sheet and brush with egg wash. Repeat, filling 2 greased baking sheets in all. Bake knishes until golden brown, about 30 minutes. Let cool slightly; serve warm.