Originally Posted by LapLap

Do you know what the “red mustard” is in Japanese. It’s not a tsukemono vegetable/herb I’m familiar with and I can’t work out what it would be.

Edit - I suspect it’s 赤大葉高菜（あかおおばたかな) Aka Obaa Takana, Red (big leaf?) Takana.
I only know Takana as green, perhaps the red is best pickled in some way as red shiso (red perilla) is.