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).