Can I fix a chewy beef joint after it's already cooked and sliced?
Stack the slices up neatly, tie them together with some string and either use a slow-cooker (crockpot), or steamer to finish the cooking

Cook for at least the amount of time it missed on plus about 5 to 10 minutes for it to get back up to cooking temperature

The other option is to cut it up and add use it to make a new stew or casserole