Der Feuervogel aufersteht.

It may be okay as is... it reads as being given and taken as an insult, so most readers will probably just accept it. It feels like an explanatory aside there would break the pacing. Now, if you could work the du/sie explanation into some *earlier* scene, perhaps with some Hessian explaining it to a non-native speaker off hand, then readers who remembered that could feel clever.