Combine the oil, mustard, whiskey, soy, ancho chile, red wine vinegar, brown sugar, onions, garlic and black pepper in a medium bowl. Place the steak in a large baking dish, pour marinade over and turn to coat. Cover and marinate in the refrigerator for 1 to 2 days.
Preheat grill to high. Remove steak from marinade, season with salt and grill for 15 to 20 minutes or until medium rare. Remove from the grill and let rest 10 minutes before slicing. Serve with relish on side.
Combine all ingredients in a medium bowl. Serve at room temperature.