An expensive delicacy found only in the Guardia region. It's sort of like truffles without the chocolate.
The Mushroom is a rare, expensive delicacy from the region of the Kingdom of Guardia on Zenan. One such specimen was carried by an explorer and his son on their travels to find more mushrooms; while in El Nido, they stopped at Shadow Forest, and the man became stuck and threatened by the monsters dwelling in the waterway between the forest and Viper Manor. Serge saved the man, and he gave Serge one of the Guardian mushrooms. Serge later presented it to Lisa's father, who, upon eating it, turned into Funguy and joined the party in hopes of having the Frozen Flame restore him to human form.
From: Key Items (Chrono Cross)