In a large salad bowl mix together cabbage, onion and sugar.
In a small saucepan over medium high heat add vinegar, vegetable oil, salt, celery seed and mustard. Bring to a boil and then simmer for 2 minutes. Remove from burner and allow to cool down.
Once cooled down, pour vinegar mixture on top of cabbage in bowl and toss to completely coat.
Cover with plastic wrap and refrigerate overnight for maximum flavor.
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Notes
Tip: You really need to refrigerate this overnight to get the full taste! Try to plan ahead, your tastebuds will thank you!