After defeating Mustard and receiving your Dojo Uniform, Klara/Avery will request their Dojo Uniform from Mustard as well. But right as they are about to receive it, an extremely fast Slowpoke will come and snatch it before distributing the various articles of clothing with two other Slowpokes. When Klara/Avery requests the Slowpokes to give their uniform back, the Slowpoke will run off. Mustard will then announce the first of the three dojo trials in order to obtain the secret armor: catch and defeat all three of the Slowpoke. From there, it will be up to you to race down each of the Slowpoke. This can be difficult to do, but using your bicycle should help. Also, watch the Slowpoke closely as they will travel the same paths over and over. Defeat all three Slowpoke to obtain Klara's/Avery's entire uniform back.
Galarian Slowpoke x3 (100%): Level 12/60
Now that you know about Mustard's First Trial, Click Here to learn about Mustard's Second Trial!