After returning Klara's/Avery's Uniform to them at the Master Dojo, Mustard will announce the second of the dojo Trials. This one will revolve around searching for and finding Mega Mushrooms in order to make some Max Soup. Since you are new to the Isle of Armor, Mustard will show you where to go to find the Max Mushrooms. Just past the Soothing Wetlands, you will find a cave called Warm Up Tunnel where the Max Mushrooms can be found. Unfortunately, just as you find and click on the Max Mushrooms, Klara/Avery will arrive and challenge you to another battle, with the winner getting to pick the Max Mushrooms.
Now that you know about Mustard's Second Trial, Click Here to learn about Mustard's Third Trial!