You can start this quest once you reach the main quest, La Cage au Fou, and after that point can head to the house anytime to get the key. Head inside Trastamara Estate Ruins and search around until you find the Spoon key, and read the note attached to it, which will point you to the Pheasantry in western Beauclair. Head there and talk to the innkeeper about the spoon key. She won't know what it's for, but will point you towards the Pheasantry's cellar, where the past owners kept some eccentric stuff and there could be clues about it there.
Search the cellar to find Smigole Louis Serkis' cookbook, which will give you the final location, Danamebi Temple Ruins, which is north-northeast of Basane Farm. Around the north side of the ruins are steps that go down to a locked door which can be unlocked with the spoon key. There are several shelves inside you can loot, but the main item is the chest in the middle of the room. Interact with it to complete the quest.
- While snooping about the cursed Trastamara Estate, Geralt happened on a strange, spoon-like key. Attached to it was a note.
- The note labeled the strange spoon as the property of the Pheasantry, known far and wide to be the finest tavern in all the duchy of Toussaint.
- The witcher decided to follow the lead and headed to the Pheasantry to see what he could find. Though the innkeep denied ever having seen the peculiar utensil, she did suggest that Geralt search the tavern's cellar. For it was there that the previous owner's documents were stowed.
- In the cellar Geralt found some notes. These in turn stated the spoon was a key to the studio of a legendary elven cook. The studio supposedly lay hidden amidst the ruins of a temple to the goddess Danamebi in the Blessure Forest...
- Geralt reached the temple ruins, descended into the cellar, and found the long-forgotten studio-kitchen of one Ra'mses Gor-Thon, where treasure was as plentiful as dust and cobwebs!
- Read the note attached to the spoon.
- Ask the owner of the Pheasantry about the mysterious spoon.
- Search the Pheasantry's cellars.
- Read the notes found in the Pheasantry.
- Find Gor-Thon's studio and look for treasure.