Unless you are ready to eat the WHOLE ox, it should be: nous mangeons DU boeuf!!!
if you are in a restaurant and there is a choice between "beef" and "chicken", some may say "I'll have the beef" as opposed to "the chicken".
The question is 'We are eating the beef'. As in, right now, we are currently eating the beef. Maybe in a restaurant if you received a phone call asking what you were eating, you'd say that, but otherwise, it's always "nous mangeons du boeuf'.
what about "Nous mangeons de la viande de bœuf"?
It is correct for real life usage, but it does not translate "le boeuf". Actually, you used the partitive "de la viande" and not "la viande" (some meat vs the meat).
RIgoletta is correct, and so am I , and the translation is incorrect and lame.
SItesurf is also correct, but we have no context in this one lone sentence.
I thought I had it right. It is du boeuf