I’m glad you’ve decided what to do with your beef mumski. It will be delicious.
I did beef, too, and although not overcooked it was more well done than medium (and I prefer it slightly pink). Still nice, though, and not tough. However, there was quite a lot left, so I minced it up and divided into two portions. One portion I used to make a cottage pie which we had day after Boxing Day. The other half I’ve put in a box and frozen to use another time - either for another cottage pie or perhaps for a chilli con carne.