Line a large, rimless baking sheet with parchment paper or grease with butter. Place instant potatoes in a medium bowl. Add boiling water and stir to combine. Measure 2 teaspoons melted butter and set aside. Add remaining butter to potatoes and mix well. Stir in fried onions and 1/4 cup parsley. Mash fish and add to potato mixture. Add salt and pepper to taste. Mix well.
Remove both pieces of puff pastry to a floured board. Unfold and cut along natural folds to form a total 6 equal rectangular pieces (from both sheets of puff pastry). Remove 2 pieces of pastry and set aside for another use. With a floured rolling pin, roll remaining 4 pieces slightly to flatten.
Spoon 1/4 of potato-fish mixture onto each of the 4 puff pastry pieces and make a log of the fish mixture down the center. Roll each puff pastry piece into a log (around fish mixture) from longest side (like a jelly roll). Pinch seam lightly to seal. Trim off 1-inch of unfilled dough ends. Place logs seam side down on prepared baking sheet and brush with reserved melted butter. Sprinkle with fish seasoning. Using a serrated knife, cut each log into 8 equal pieces and separate and place them 1/2 inch-apart on the baking sheet. Bake in a preheated 375 degree F oven 30 to 35 minutes until golden brown.
To serve, arrange knishes on a serving platter. Garnish each piece a small dollop of horseradish sauce with lemon. Sprinkle with chopped parsley and serve as an appetizer (finger food).