Here’s how I reheat med-rare beef in any form. First, and importantly, warm the meat to room temperature. It’s extremely easy to over cook it with reheating, if you’re start from fridge chill! Second, slice the meat thin ish. Not cold cuts thin, but not thick slabs either!
Next prepare all the side dishes, while bringing almost to a boil, either thinned gravy (possibly leftover?), or Au Jus. Just before serving simply dunk the meet slices into the heated gravy/au jus, briefly and then immediately plate.
Personally I wait till I have everything else plated before I dunk the leftovers into the reheating sauce. This works remarkably well, the beef is delicious and not even a titch over cooked.
It’s even easier than Sous Vide, you don’t need to get the meat up to its original cooking temperature, just warm enough that it’s at serving temperature. Just get your hot tap water to as hot as it will go first, then fill up a pot with hot water and drop your steak in inside some sort of waterproof pouch. Leave it in there 5 - 10 minutes and it will be ready to eat.
That’s what I was trying to say less eloquently above. When I was a cook in a restaurant, that is how we heated meat for use on our steak and cheese sandwiches and for our steak salads. The meat was the left over cuts of prime rib from dinner the night before. You’re correct that the meat should be cut thin and heated just before its plated.