Preheat oven to 350 F. Divide dough in two. Roll out each section to a 12 by 12 inch rectangle. Apply half the mashed potatoes and roll into a log. Cut in 15 sections. Turn cut side down of each knish and press down slightly.
Place knishes on a parchment-lined baking sheet.
Spray knishes with non-stick cooking spray or brush with beaten egg. These freeze well.