The recipe isn't actually on any of the merchants - there's a drunk mage in one of the corners (opposite side that the bar is on) with a quest !over his head - click on him and it'll teach you the recipe (assuming you've already done the Nessing quest).
You can find the fruit to make it being sold by the barmaids on the other side of the sewers.
The fish feast cost 5 cooking awards and is on the last page, the new worg recipe is on the second page and costs the normal 3.