First of all, slice the beef and allow it to sit out and come to room
temperature. This will not cause any effects from "cooking". If you
have any gravy, heat it very hot, and pour over the beef. If not, heat
it in small increments,covered, in the microwave, let it sit, test the
temp. I wouldn't put it in the oven--I think you can control it better
in the microwave.