I'm not necessarily a ketchup person either, but I tried this kinda no name one and it's actually pretty good.

It's got lots of tomato and some sugar, I've seen ketchups that have straight up corn syrup in them, not in this one.

For this sandwich it was an absolute must and a much better choice than say mayo...

Because that minced meat is pretty dry and Asian veggies beg for sauce...

Probably other good options would've been BBQ, sweet chili or Oyster sauce, or some other cool Asian sauce.