Collect Celestine Caverns Mushrooms in Celestine Caverns. When you have enough take them to Alchemist Isaac. Use your quest tracker to see your progress.
Alchemist Isaac: Crayfish is changing my life! I'd like to name my next Alchemist Isaac: discover after it. However, I can't live off of this alone! HiiragiKagami: Then why not eat the food provided by the military? Alchemist Isaac: Clearly, that's not an option. Not after this delicacy! NakanoAzusa: Are there other things to eat? Alchemist Isaac: Yes, luckily there are edible mushrooms in the cave. Alchemist Isaac: They're not as good as farmed mushrooms, but I can Alchemist Isaac: sprinkle some alchemy on it! NakanoAzusa: I think I've seen them, but I thought they NakanoAzusa: were poisonous. Alchemist Isaac: Yes, they look similar to Lakia fly mushrooms, don't Alchemist Isaac: they? Now hurry along!
Reminder Dialog
Alchemist Isaac: There are monsters looking for the mushrooms as Alchemist Isaac: well. Be careful when you gather them.