This page explains how to start and complete the Curry for What Ails You side quest.
How to Start the Quest
Talk to Lester at Rito Stable (near Rito Village) to start the quest. He asks for Goron Spice.
How to Complete the Quest
Go to Goron City and purchase Goron Spice from the shop there. Give it to Lester and he will give you a purple rupee. After that, you can trade him Goron Spice to get Hylian Rice.