In a small bowl, whisk together 3 tablespoons of olive oil, red wine vinegar, sherry vinegar, mustard, and dried thyme. Season the steaks generously with salt and pepper, rubbing the seasoning into the meat. Place the steaks in a gallon sized zip-top bag, pour the marinade over the steak, and massage it into them, ensuring they’re evenly coated. Seal the bag, getting any excess air out, and let marinate in the fridge for at least 30 minutes, preferably overnight. Prior to cooking, remove from the fridge and allow to come to room temperature on the counter for at least 30 minutes.