Heat a skillet over medium high heat. Add oil and onion and saute 2 minutes. Add cabbage and turn in pan, sauteing it until it wilts, 3 to 5 minutes. Add vinegar to the pan and turn the cabbage in it. Sprinkle sugar over the cabbage and turn again. Season with mustard seed, salt and pepper and reduce heat a bit. Let cabbage continue to cook 10 minutes or until ready to serve, stirring occasionally.