Add onion and red pepper to 6 cups of the cabbage base.
Bring the sugar, vinegar, Dijon mustard, salt, pepper and celery salt to a boil in a microwaveable dish. Remove from microwave and add the oil.
Pour the hot dressing over the cabbage mix and stir well.
You can serve it immediately (it will be room temperature when the dressing mixes with the cabbage). You can chill it in the fridge and serve cold. The flavours do develop over a day or so in the fridge.
To freeze - after salad has been refrigerated for a day or so transfer the slaw to freezer containers in what ever portion sizes you want and freeze it.
Thaw in fridge when ready to use. Serve with a slotted spoon since more liquid does develop over time.