Even if you like to cook beef all the way through, this "post-marinade" (which you add after cooking rather than before) will help add moisture to the meat at the end, as well as flavor. A post-marinade is also great for times when you forget to plan ahead!
Technique tip: When cutting the meat, cut against the grain. If you cut with the grain, the meat will be chewy.
For the steak:
Season the flank steaks with the thyme and black pepper. Cover and refrigerate overnight or all day.
For the post-marinade:
Whisk together vinegar, olive oil, garlic, and a pinch of sea salt and set aside at room temperature.
To cook and serve:
Preheat a grill to medium heat, about 400°F. Brush both sides of meat with olive oil and sprinkle with sea salt.
Grill the meat until desired the doneness is achieved, 3 to 4 minutes on each side for medium, turning with tongs, not a fork. Transfer to a serving platter and pour the post-marinade over the meat. Cover with aluminum foil and allow to rest for 5 to 10 minutes.
Transfer the meat to a cutting board and slice thinly against the grain, which means you are cutting in the opposite direction of the lines in the meat. If you cut with the grain, the meat will be chewy. Place back on the platter and serve.
Excerpted from Kitchen Matters: More than 100 Recipes and Tips to Transform the Way You Cook and Eat—Wholesome, Nourishing, Unforgettable by Pamela Salzman. Copyright © 2017. Available from Da Capo Lifelong Books, an imprint of Perseus Books, LLC, a subsidiary of Hachette Book Group, Inc.