Preheat the oven to 375°F (190°C).
Boil the potatoes in a pot of water until tender, then drain and mash them.
In a pan, sauté the diced onion in olive oil until soft and translucent. Mix the onions into the mashed potatoes and season with salt and pepper to taste.
Roll out the puff pastry dough into 4 equal-sized squares or rectangles.
Divide the potato filling evenly among the dough squares, placing it in the center of each square.
Fold the dough over the filling to form a rectangular parcel, pressing the edges to seal.
Place the knishes on a baking sheet lined with parchment paper. Brush the tops with the beaten egg to create a golden crust.
Bake in the preheated oven for about 25-30 minutes, or until the pastry is golden brown and crispy.
Let cool slightly before serving. Enjoy!
