in medium bowl, toss together cabbage, chile, lemon juice, salt, and sugar. taste and adjust seasoning. you are looking for a well-balanced, sweet-and-sour taste.
heat oil in a small skillet over high heat. when oil begins to smoke, add mustard seeds, covering pan. when seeds stop popping, immediately pour oil over cabbage salad and toss well. let salad sit for at least 15 min before serving.