From the Super Mario Wiki
A Trial Stew is an item in Paper Mario: The Thousand-Year Door and Super Paper Mario, made by cooking Couple's Cake with a Poison Shroom. If used, it lowers Mario and his partner to one HP and down to zero FP, but completely fills Star Power.
Lowering the player to one HP could offer an opportunity for a Mega Rush P Strategy, but this strategy hits a roadblock because the FP is lowered to zero. Some players ignore this and don't use attacks that consume Flower Points. Others use Sweet Treat or Sweet Feast, or an FP-restoring item, to bring their FP up (however, they have to be careful not to up their HP unless they want to use Power Rush instead, in which they can up their HP to five). There are many strategies for using Trial Stew, but the casual player will probably have no use for this, other than refilling star power, which is useful if the player is running low on star power.
In Super Paper Mario, the Trial Stew causes the player to lose all but one HP, but increases their point total by an amount equal the amount of HP lost times 100.
Names in Other Languages
Shiren no Nabe