Thinly slice the onions and separate them into rings.
In a large bowl, alternate layers of the cabbage and onion rings.
In a small saucepan, combine vinegar, mustard, salt, celery seeds and cornstarch.
Stir to dissolve cornstarch.
Cook over medium heat until mixture boils and is clear.
Remove from heat, beat in sugar replacement and oil.
Pour dressing over the cabbage.
Cover tightly and refrigerate 8 hours or overnight, stirring occasionally.