Help - my beef is tough when reheated

[doodle]

Thanks to everyone for your suggestions of other options to re-heat the rest of the frozen batches. I have never had this happen before and you have all given me thoughts as to what went wrong.
I suspected that the re-heat probably created a boil because it sat in so much liquid and this meat was sliced far too thin. I also was wondering if I had made the jus more of a (thicker) gravy or if I had heated the thawed jus in a pan and wrapped the thawed meat in foil & heated it separately that would have made a difference? The reason it was in the oven for 1 hour is because it took that long to heat it through. Clearly I need to revamp my method. I thought that b/c I froze the meat in a medium rare condition that would add to the success of the finished product to tender pot roast servings.
Anything you have to further enlighten me would be greatly apprecialted.
much thanks- doodle

[Dave Hutchins]

round of beef is notorious for being tough, I would very briefly heat it in some warm Au Jus put in a sub bun and dip in Jus and enjoy. Do not heat for a long period as it will be way to tough to eat, do not boil awfullllllllllllllllllllll.

[Jeff G.]

Easier re-heating.

Bring some beef broth up to simmer. thaw the slices and quickly dunk them in the broth.

[Jeekinz]

For a leftover roast, let stand until room temperature and slice. Heat up some broth or make a sauce or au jus and use that liquid to heat the meat. Works for leftover pork too.