Gather the ingredients.
Toss cabbage and onion together in a large bowl; set aside.
Combine sugar, vinegar, oil, celery seed, salt, mustard, and pepper in a small saucepan; bring to a boil and simmer, stirring frequently, until sugar is dissolved, about 3 minutes.
Let mixture cool slightly, about 5 minutes, then pour over cabbage mixture; toss well to coat.
Serve and enjoy!
