I have never in my life heard of a kohlrabi, so I guess I learnt something new. Also, I feel like this could have just asked for katsudon instead of trying to awkwardly make tonkatsu a k word.
I've never heard of tonkatsu--seems like knackwurst would be way more famous! Anyway, one time my mom served us some "potatoes." We dug in. Yuck. She had tried to trick us by cooking up some kohlrabis and saying they were potatoes. It took me quite a few decades to forgive this vegetable for its part in the ruse.