Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius). Line a non-disposable baking sheet with Gefen Parchment Paper.

Place all filling ingredients in a large bowl. Use a potato masher or a large fork to mix everything together until potatoes are smooth.

Lay out a puff pastry square. Scoop about two tablespoons of the potato-spinach mixture over half of the square. Fold the corner of the dough over the mixture to create a triangular shape. Use a fork to press down on the edges of the dough and seal. Then poke a small hole on the top of the turnover. Repeat with remaining pastry squares.

Place the filled knishes on the prepared baking sheet and brush with beaten egg. Place in preheated oven and bake for 45 minutes, or until golden brown.