In a large, sealable plastic bag, combine the vinegar, oil, garlic and thyme. Add the steak, making sure to coat both sides with the marinade. Press out at air in the bag and seal it tightly. Place on a plate (in case of leaks) and refrigerate for at least 4 hours, preferably overnight. Allow the steak to come to room temperature before grilling.

Oil and preheat the grill or skillet. Place the steak on the hot grill or skillet over medium-high heat, brushing the topside of the steak with marinade. Cook one side for 4 minutes; using tongs, flip the steak and brush with a little marinade. Cook the other side for 4-5 minutes for medium-rare, a minute or two longer for medium (Discard leftover marinade).

Transfer the cooked steak to a cutting board. Allow to rest for 5 minutes. 510 calories/serving; GL=0, Glycals=0