Whisk together the balsamic vinegar, olive oil, garlic, and thyme. Alternatively, add those ingredients to a blender. Blend until smooth. Pour the balsamic marinade over the steak and allow to marinate for at least 30 minutes, ideally overnight in the fridge. The longer you marinate the steak, the more flavor it will be and it will also be more tender.
Season with steak well with salt and pepper. Grill for 6-8 minutes per side for medium or test with an instant read thermometer and cook until 135-145°F. Let the steak rest for at least 5 minutes and thinly slice to serve.