- Acquire 5 meals of Kept Sour Ambrosial Meat.
Ambrosial Meat is dropped by defeating a Griffin, or one of the many oxen that wander Gransys. Once you have the meat, let it mature in your inventory until it is "sour." Then, use an Airtight Flask to preserve it, thus making it a Kept Sour Ambrosial Meat.
It is recommended that you keep the Airtight Flask in your inventory so that you can preserve the valuable meat the moment it goes sour. Do not give it a chance to rot.
Sour Ambrosial Meat can be found as a possible loot in crates in The Ancient Quarry. The crates in the main hallway, just east of the ! Lever (the middle ! on the map) can drop the meat. Then all you need to do is preseve it in an Airtight Flask. The crates and their contents respawn every 10 days, so a possible solution would be to rest at the Resting Campsite in West Gransys for 10 days.
To obtain Ambrosial Meat from a Griffin, one strategy is to purchase Airtight Flasks from Camellia in Gran Soren, sleep until daylight at the Gran Soren Union Inn, and continue to leave the city to see if the Griffin spawns, repeating trips to the inn as necessary. The Griffin seems to spawn on bright sunny days near the Harpy pond in Estan Plains. It can be seen often picking off oxen.
Another method is to slaughter the oxen and pigs in the pens near the western gate of Gran Soren, the one with the large field. The ox and pigs have a moderate chance of dropping ambrosial meat in addition to the standard steaks. Then all you have to do is wait and check the meat, putting them into the flasks once they turn sour but before they rot.
You can also use the "Supply and Demands" quest as an alternative to obtaining Ambrosial Meat from drops. When doing the "Supply and Demands" quest simply talk to Julien. You will then obtain the meat that you need. Take the meat to a forger and have it forged. Sour the forged meat and complete this quest.