This word uses the on'yomi of 豚. You haven't learned that yet, so here's a mnemonic to help you:
You really like breaded pork cutlet so you just ate a whole ton (とん) of it. Imagine how you feel after eating a ton of tonkatsu.
There aren't many tonkatsu restaurants around here.
I think I'll have the breaded pork cutlet set meal.
Ahhh, why did I eat so much tonkatsu yesterday?