In a food-safe bag add all of the marinade ingredients
Add the flank steak to the bag and carefully massage slightly to ensure the ingredients mix and coat the steak
Let the steak sit at room temperature in the marinade while your sous-vide heats up
Set sous-vide to 130°, or just below the final temperature you like your steak cooked
Once the water is at temperature, carefully vacuum seal the bag or use the displacement method, putting the steak into the water
Leave in the sous-vide for anywhere from 1.5 to 4 hours
Just before you pull the steak out, heat a cast iron pan or grill pan on high, pull the steak out of the bag and put it in the pan searing both sides for approximately 45 seconds or until char marks form
Place the steak on a cutting board and cover with foil, let rest for 5 - 8 minutes
Slice the steak thinly against the grain
Notes
If you don't have a sous-vide you can marinate the steak overnight and then grill on the BBQ or cook in a cast iron pan. Cook the steak to your desired temperature. We like it still pink, but if you sous-vide it it will still be moist even if you like it medium or medium well.