look for a dish called Cha Ca La Vong or Cha Ca Ha Noi. It's a collision of fish, tumeric root and dill that will blow your mind.
And Bun Cha Ha Noi is no slouch, either. There's also a really cool place in Old Town Hanoi ... I never knew the name of it, but if you see what looks like an outdoor Mongolian BBQ complete with skinned frogs, you're in the right place.