The Korok Trials
Talk to Chio after beginning the main quest "The Hero's Sword". You can learn more about "The Hero's Sword" here. Chio is standing on a stump in the Great Hyrule Forest near Kula and Peeks.
Chio will have three trials prepared for you that all Koroks undergo as a rite of passage. To complete this sidequest, you will need to complete three Shrine Quests. Note that even if you have unlocked and completed the Shrines they are connected to, this sidequest will not be complete until the three Shrine Quests are clearly marked as completed.
Click on each Shrine Quest name to go to their respective page:
Once you have completed all three Shrine Quests, speak with Chio again. Even if you have unlocked all three Shrines on your own, you cannot complete this sidequest until those three Shrine Quests are explicitly marked as done on your Adventure Log. Conversely, you can speak with Chio to finish this sidequest even if you have not completed the three Shrines.
As a reward, Chio will gift you three Big Hearty Truffles.