Witcher Seasonings - Hearts of Stone
This quest begins during the Open Sesame Hearts of Stone Story Quest. You must complete this quest in order to complete the Open Sesame quest. For this quest you'll first be tasked with creating the Cleansing Mixture (pictured above) then heading over to Oxenfurt where we will be completing the quest.
You'll need to use your Witcher Senses on 3 different sets of guards to eavesdrop on them. Just get close enough to them (you don't have to hide at all) to hear what they're talking about, the third pack of guards you eavesdrop on will give you the information you need and your quest will update.
Travel to the southwestern pier of Oxenfurt during the day to meet the cook, Maarten. You'll either be able to pay him 500 Crowns to spike the food or use Axii on him and have him do it for free. Paying him will net you a slightly larger EXP reward but otherwise there's no difference.
Once you've spoken with Maarten the quest will conclude here. Return to my Open Sesame! Walkthrough to continue with the main quest or head on over to Open Sesame: Breaking and Entering or Open Sesame! The Safe Cracker for more information about these quests.