Forbidden Fruit is a side quest from the 
Wedding Crashers questline and a part of the Kreyzl route to the wedding. First, complete Materia Prima in 
Lower Semine Mill, Kreyzl will then offer Forbidden Fruit and the last quest in the row is Opus Magnum.
Hidden Lair
The Forbidden Fruit side quest is started by speaking with Kreyzl and then Bonnie in the 
The Lair interesting site.
If you bring Bonnie herbs, she rewards you with a small reputation boost. She explains the task of finding saltpetre pits, which begins the investigation.
Preparing for the Trip
Your first destination is the Troskowitz Cemetery.
Before heading there, stop by the Troskowitz butcher and buy some meat. Three pieces of bacon work well.
Visit his house and explain the situation. Regardless of your dialogue choices, the quest requires you to help him. Return early in the morning to start the trip on the cart.
First Stop: Ruined Building
The first stop is the Ruined Building (Troskowitz).
There is little to find here besides a Marigold Decoction and an axe. Enter through the back window and open the door from inside. This lets you retrieve the carcass without needing to pick any locks.
When you return with the carcass, Ignatius is confronted by several men. You can avoid a fight by choosing the Coercion (Snake tongue) dialogue option.
Second Stop: Plagued Farm in Schdiar
The second stop involves carrying two pig carcasses and optionally investigating the source of a plague near the Schdiar - West Farm by simply going to the marker near the water hole. This reveals the source - a dead body in the stream.
Searching the area reveals a deer hunting spot, but there are no major rewards tied to this location.
Third Stop: Slatego
The final stop is Slatego. Ignatius asks you to sneakily kill a dog but upon finding the dog house, you'll discover she is gone.
Follow the trail and find that, funnily enough, it was Bonnie who stole him. If you want to save the dog and preserve your Lent achievement, you can simply let her keep it. Choose the dialogue options:
"I asked you first" then "I don't want to kill him". Bonnie will then take the dog to The Lair. Ignatius will be unhappy and your reputation will drop, but the quest still continues normally.
Saltpetre Pit
After finishing the investigation, the quest moves you to the saltpetre pit.
Ignatius rewards you with Knacker's Gloves, and you can taste the saltpetre during the scene. You do not need to travel further with him and can call your horse from this location.
Return to Kreyzl and report what you discovered. He rewards you with the 
As Quiet as a Cat II skill book and 85 Groschen. This completes the Forbidden Fruit quest.
Forbidden Fruit FAQ
How do I start the Forbidden Fruit quest?
First complete Materia Prima in Lower Semine Mill. Kreyzl will then offer Forbidden Fruit. Speak with Kreyzl and then Bonnie at The Lair interesting site to start the quest.
Where is the first stop in Forbidden Fruit?
The first stop is the Ruined Building (Troskowitz). Enter through the back window and open the door from inside to retrieve the carcass without lockpicking. Use Coercion (Snake tongue) to avoid a fight when returning.
How do I save the dog in Forbidden Fruit for Lent?
At Slatego, when you find Bonnie stole the dog, choose "I asked you first" and "I don't want to kill him". Bonnie will take the dog to the Den. Your reputation drops but the quest continues and your Lent achievement is preserved.
What are the Forbidden Fruit quest rewards?
Kreyzl rewards you with As Quiet as a Cat II skill book and 85 Groschen. Ignatius gives you Knacker's Gloves at the saltpetre pit.
Is Forbidden Fruit on the Kreyzl wedding route?
Yes. It is a Wedding Crashers side quest on Miller Kreyzl's path. Complete Materia Prima first, then Kreyzl can offer Forbidden Fruit before you secure your invitation.