Preheat oven to 400° F Prepare a sheet tray with parchment paper. Take out your leftover mashed potatoes that are chilled. Saute the minced onion until soft. Take out the puff pastry. On one third of the pastry make a line across vertically with the mashed potato. Roll up to the crease and slice the puff pastry across. Seam up the seam and place seam side down on the sheet tray. Repeat until you run out of potatoes and puff pastry. Score the top with a knife to show where you want to cut the knish. Brush with the egg wash of the egg and a teaspoon of water. Bake for 15-18 minutes until golden brown. Enjoy with a glass of wine. Bon Appetit.