Remove the meat from the refrigerator about an hour before cooking. Wipe it with paper towel.
Mix the oil, vinegar, thyme leaves and garlic. Rub the steaks with the marinade, then rest.
Heat a griddle or frying pan. Add the steak, together with the marinade, and cook for about 2- 3 minutes on each side for medium, or longer if you like it well done.
Transfer the steaks onto a plate and let them rest (covered) for 5 minutes.
Mix the yogurt with the mustard (optional).
Serve with mashed potatoes or French fries - with the yoghurt & mustard sauce if you like.