Thaw frozen knishes completely. Preheat oven to 375-400 (as ovens vary). Prepare egg wash (definitely preferred but optional). Egg Wash: whisk one egg yolk alone OR with one tablespoon of water or one tablespoon of milk, depending on your preference and/or dietary restrictions. This should be enough for the four knishes. Place knishes on foil or parchment paper on a cookie sheet and brush the tops and sides with the egg wash. Bake in oven for 25-30 minutes (as ovens vary) or until top is golden brown as desired and filling is hot. Be cautious at first bite as filling could be very hot.