Medium-rare meat is purely a function of internal temperature.
On a grill you try to cook from the outside to get the inside to the appropriate temperature.
Sous-vide is a easy cooking style that ensures the internal temperature is correct, independent of time or thickness of your steak (or any meat really).
You then finish on a really hot grill to get the Maillard reaction which is the browning of the amino acids and sugars in the steak as quickly as possible, trying not to further heat the inside.
- chilly bin
(yes, these work with hot water too)
- zip lock glad-bags
- add a bunch of hot water and or boiling water to the chilly bin and ensure it's somewhere between 55 and 60 celsius for medium rare
- put steak in zip lock glad-bags or similar and try to get all the air out - you can add a bit of salt & pepper or garlic or other seasoning, but I've found the flavour is quite strong
- put steak-bags in water - if they don't sink, let air out of them carefully
- keep an eye of the temp and add a bit more water appropriately
- after about 45 mins they are ready and you could technically eat them. You can also wait longer as you can't over do this unless the temperature rises. It also doesn't matter if the temp drops a bit as long as it's had at least 45 mins at temp, so you can just wait till you are ready to serve and get everything else prepped
- grill the steak on the hottest grill you can for the shortest length of time you can to achieve a nice crisp brown exterior without heating the inside of the steak
- You can see in the cross section of the meat after you've cut it, the gray gradient coming in from the edges is the heat from the grill - ultimately you want NO gray and to go from crisp brown to nice pink meat.