Step 1

Half the cabbage, remove the core, and thinly slice it. Place in a large bowl.

Step 2

Peel and thinly slice the onion. Add it to the cabbage.

Step 3

In a medium bowl, whisk the vinegar, oil, salt, sugar, dry mustard, and celery seed together. Pour the dressing over the cabbage and toss to coat.

Step 4

Cover and refrigerate for at least 4 hours, but preferably overnight - tossing occasionally to coat the slaw in the dressing. Taste and add salt and pepper to taste before serving.