Preparing the Potage Le Magnifique
This quest is actually very simple. You have already got the required paperwork to get inside the palace. In the making of the Potage Le Magnifique you can add anything you want to the dish, Gianna won't question it since you are the Gourmet. At the end be sure to add the Jarrin Root, it will save you from manually killing the Emperor.
Killing the Impostor of the Emperor of Tamriel
If you have added the secret ingredient (Jarrin Root) the the impostor of the emperor of tamriel will die automatomatically. To speed things up you can use other means of killing him. The only thing that is important is that he dies.
Escaping the Palace Without a Bounty
After you leave the death scene you will run into 4 imperials who will try to put you into prison. They are easy to kill, although I advise against using a bow and arrow because a sword can deal out much more damage per time than a bow unless you are sneaking. You won't have a bounty after you have killed all four guards. All of the enemies are called "Penitus Oculatos Agent" and they are much weaker than normal guards are.
After you have escaped you have to head back to the Dark Brotherhood Sanctuary that has been raided. At this point the next quest starts.
