Do I need to use fresh horseradish or can I use Gold's from the jar?

As long as you are using just a plain prepared horseradish and not the sauce, I would think it could work just fine. Since the jar horseradish already contains vinegar, I wouldn't add in extra cider vinegar though. Given that this is for Passover, I would also purchase a new jar certified Kosher for Passover as well - not the one in the fridge that is months old. If you wanted the slight sweetness of cider vinegar, you could also toss in a slice of green apple to the processor as well. If you decide to do this, let us all know how it turns out, would you?? I think the recipe sounds delicious. I'm not Jewish, but recipes for a good roast chicken dinner are always welcome.